North American Numbering Plan

The North American Numbering Plan (NANP) is a telephone numbering plan for World Zone 1, which comprises twenty-five distinct regions in twenty countries primarily in North America, including the Caribbean. Some North American countries, most notably Mexico, do not participate in the NANP.

"NANPA" redirects here. For other uses, see Nanpa (disambiguation).

The NANP was originally devised in the 1940s by AT&T for the Bell System and independent telephone operators in North America to unify the diverse local numbering plans that had been established in the preceding decades. AT&T continued to administer the numbering plan until the breakup of the Bell System, when administration was delegated to the North American Numbering Plan Administrator (NANPA), a service that has been procured from the private sector by the Federal Communications Commission (FCC) in the United States. Each participating country forms a regulatory authority that has plenary control over local numbering resources.[1] The FCC also serves as the U.S. regulator. Canadian numbering decisions are made by the Canadian Numbering Administration Consortium.[2]

The NANP divides the territories of its members into numbering plan areas (NPAs) which are encoded numerically with a three-digit telephone number prefix, commonly called the area code.[3] Each telephone is assigned a seven-digit telephone number unique only within its respective plan area. The telephone number consists of a three-digit central office code and a four-digit station number. The combination of an area code and the telephone number serves as a destination routing address in the public switched telephone network (PSTN). For international call routing, the NANP has been assigned the international calling code 1 by the International Telecommunications Union (ITU). The North American Numbering Plan conforms with ITU Recommendation E.164, which establishes an international numbering framework.[4]

History
Area code handbook by the Bell Telephone Company of Pennsylvania issued in 1962 to promote the newly introduced direct distance dialing

From its beginnings in 1876 and throughout the first part of the 20th century, the Bell System grew from essentially local or regional telephone systems. These systems expanded by growing their subscriber bases, as well as increasing their service areas by implementing additional local exchanges that were interconnected with tie trunks. It was the responsibility of each local administration to design telephone numbering plans that accommodated the local requirements and growth.[5] As a result, the Bell System as a whole developed into an unorganized system of many differing local numbering systems. The diversity impeded the efficient operation and interconnection of exchanges into a nationwide system for long-distance telephone communication. By the 1940s, the Bell System set out to unify the various numbering plans in existence to provide a unified, systematic approach to route telephone calls across the nation and provide efficient long-distance service that eventually did not require the involvement of switchboard operators.

A new nationwide numbering plan was officially published in October 1947 in cooperation of the Bell System with the independent telephone operators. The plan divided most of North America into eighty-six numbering plan areas (NPAs). Each NPA was assigned a unique three-digit code, typically called NPA code or simply area code. These codes were first used in Operator Toll Dialing by long-distance operators in establishing calls between toll offices. The first customer-dialed direct call using area codes was made on November 10, 1951, from Englewood, New Jersey, to Alameda, California.[6] Direct distance dialing (DDD) was subsequently introduced across the country. By the early 1960s, most areas of the Bell System had been converted and DDD had become commonplace in cities and most towns in the United States and Canada. By 1967, the number of assigned area codes had grown to 129.[7]

The status of the network of the 1960s was reflected in a new name used in technical documentation: North American Integrated Network.[7] By 1975, the numbering plan was referred to as the North American Numbering Plan,[8] leading to the well-known acronym NANP, as other countries sought or considered joining in the standardization.

Foreign expansion

Although Bermuda and the Caribbean islands had been assigned the area code 809 as early as 1958 by the administrators at AT&T, individual participating countries or territories had no autonomy over their numbering plan as they received centrally assigned central office prefixes that needed to be unique from those of other countries with the same area code. Regions in Mexico with high call volumes to and from the US were assigned functional area codes as early as 1963, for the purpose of call routing, but a nationwide system of participation in the NANP eventually failed.

In the following decades, the NANP expanded to include all of the United States and its territories, Canada, Bermuda, and seventeen nations of the Caribbean.[9][10]

At the request of the British Colonial Office, the numbering plan was first expanded to Bermuda and the British West Indies because of their historic telecommunications administration through Canada as parts of the British Empire and their continued associations with Canada, especially during the years of the telegraph and the All Red Line system.

Not all North American polities participate in the NANP. Exceptions include Mexico, Greenland, Saint Pierre and Miquelon, the Central American countries and some Caribbean countries (Cuba, Haiti, the French Caribbean and the Dutch Caribbean, except for Sint Maarten, which uses the 721 area code). The only Spanish-speaking state in the system is the Dominican Republic. Mexican participation was planned,[11] but implementation stopped after three area codes (706, 903 and 905) had been assigned, and Mexico opted for an international numbering format, using country code 52.[12] The area codes in use were subsequently withdrawn in 1991. Area code 905, formerly used for Mexico City, was reassigned to a split of area code 416 in the Greater Toronto Area; area code 706, which had formerly served Mexico's Baja Peninsula, was reassigned to a portion of northern Georgia[13] surrounding the Atlanta region, which retained 404; and area code 903, which served a small portion of northern Mexico, was reassigned to northeastern Texas when it split from area code 214.

The Dutch Caribbean territory of Sint Maarten joined the NANP in September 2011, receiving area code 721.[14] Sint Maarten shares the island with the French Collectivity of Saint Martin which, like the rest of the French Caribbean, is not part of the NANP.

Administration

The NANP is administered by the North American Numbering Plan Administrator (NANPA, formerly Administration).[15] This function is overseen by the Federal Communications Commission, which assumed the responsibility upon the breakup of the Bell System. The FCC solicits private sector contracts for the role of the administrator.

Before the breakup of the Bell System, administration of the North American Numbering Plan was performed by AT&T's Central Services Organization. In 1984, this function was transferred to Bell Communications Research (Bellcore), a company created by the divestiture mandate to perform services for the newly created local exchange carriers. On January 19, 1998, the NANPA function was transferred to the IMS division of Lockheed Martin in Washington, D.C.[16] In 1999, the contract was awarded to Neustar, a company spun off from Lockheed for this purpose. In 2004, and again in 2012, the contract was renewed.[17] On January 1, 2019, Somos assumed the NANPA function under a one-year bridge contract granted by the FCC with the goal of consolidating the NANPA function with the Pooling Administrator and identifying a long-term contract holder.[18][19]

Numbering plan

The vision and goal of the architects of the North American Numbering Plan was a system by which telephone subscribers in the United States and Canada could themselves dial and establish a telephone call to any other subscriber without the assistance of switchboard operators. While this required an expansion of most existing local numbering plans, many of which required only four or five digits to be dialed, or even fewer in small communities, the plan was designed to enable local telephone companies to make as few changes as possible in their systems.

Numbering plan areas and central offices

The new numbering plan divided the North American continent into regional service areas, called numbering plan areas (NPAs), primarily following the jurisdictional boundaries of U.S. states and Canadian provinces.[20] States or provinces could be divided into multiple areas. NPAs were created in accordance with principles deemed to maximize customer understanding and minimize dialing effort while reducing plant cost.[21] Each NPA was identified by a unique three-digit code number, that was prefixed to the local telephone number. If a call's destination was within the same numbering plan area, dialing the area code was not necessary.

Existing telephone exchanges and central offices became local exchange points in the nationwide system, each of which was also assigned a three-digit number, unique within its NPA. The combination of NPA code and central office code served as a destination routing code for use by operators and subscribers to reach any central office through the switching network.[20] Due to the structure of the numbering plan, each NPA was technically limited to 540 central offices.[21]

Although the limitation to 540 central offices required the most populous states to be divided into multiple NPAs, it was not the sole reason to subdivide a state. An important aspect was the existing infrastructure for call routing, which had developed in preceding decades independently of state boundaries. Divisions also attempted to avoid cutting across busy toll traffic routes, so that most toll traffic remained within an area, and outgoing traffic in one area would not be tributary to toll offices in an adjacent area.[22][20] As are result, New York state was initially divided into five areas, the most of any state. Illinois, Ohio, Pennsylvania, and Texas were assigned four NPAs each, and California, Iowa, and Michigan received three. Eight states and provinces were split into two NPAs.

Traditionally, central office switching systems were designed to serve up to ten thousand subscriber numbers. Thus, subscribers were assigned four-digit line or station numbers. This rounded out the total number of digits in a subscriber telephone number to ten: a three-digit area code, three-digit central office code, and four digits for each line. This defined the North American Numbering Plan as a closed numbering plan,[23] as opposed to developments in some other countries where the number of digits was not fixed.

However, the closed numbering plan did not require the subscriber to dial all digits. When making a local call or a call within their numbering plan area, the area code was omitted, resulting in seven-digit dialing. Ten-digit dialing was only necessary when placing foreign area calls to subscribers in another state or numbering plan area.[24] Exceptions existed for communities located on NPA boundaries, so that uniform local dialing was still possible in historically established communities.

Initial numbering system

In 1947, AT&T completed a new design for a nationwide toll network that established the original North American area codes. The new organization provided for 152 area codes, each with a capacity to serve up to 540 central offices.[25] Originally, only eighty-six area codes were assigned. New Jersey received the first NPA code in the new system, area code 201.[26] The second area code, 202, was assigned to the District of Columbia. The allocation of area codes was readjusted as early as 1948 to account for inadequacies in some metropolitan areas. For example, the Indiana numbering plan area 317 was divided to provide a larger numbering pool in the Indiana suburbs of Chicago (area code 219).

Initially, states divided into multiple numbering plan areas were assigned area codes with the digit 1 in the second position, while areas that comprised entire states or provinces received codes with 0 as the middle digit. This rule was broken by the early 1950s,[20] as NPAs with digit 0 in the middle had to be split, but until 1995 all area codes assigned had none other than the digits 0 and 1 in this position.

The eight codes of the form N11 (N = 2–9) were reserved as service codes. The easily recognizable codes of the form N00 were available in the numbering plan, but were not initially included in assignments.[7] Additional area code patterns were later assigned for other services; for example, the area codes N10 were implemented for the Teletypewriter Exchange Service (TWX).[27]

Central office codes

It was already common practice for decades that the digits 0 and 1 could not appear in the first two digits of the central office codes, because the system of using the first two letters of familiar names for central offices did not assign letters to these digits. The digit 0 was used for operator assistance, and 1, which is essentially a single pulse of loop interruption, was automatically ignored by most switching equipment of the time.[20] Therefore, the 0/1 rule for the area code provided a convenient means to distinguish seven-digit dialing from ten-digit dialing.

The use of telephone exchange names as part of telephone numbers had been a well-established practice, and this was preserved for convenience and expediency in the new network design. The digit-to-letter translations were printed on the face of every rotary dial in the metropolitan areas, according to the scheme designed by W.G. Blauvelt in 1917, and had been used in the Bell System in large metropolitan areas since the early 1920s.[28] The network reorganization standardized this system to using a two-letter, five-digit (2L-5N) representation of telephone numbers in most exchanges in North America,[29] or to using an equivalent all-numeric seven-digit numbering plans, as was practiced by some telephone companies.

All-number calling
Partitioning of the NANP prefix space
under all-number-calling
000 — 099These 200 codes were used
as toll center and system codes
100 — 199
area
codes		service
codes		area
codes		central office
codes
200 — 210211212 — 219220 — 299
300 — 310311312 — 319320 — 399
400 — 410411412 — 419420 — 499
500 — 510511512 — 519520 — 599
600 — 610611612 — 619620 — 699
700 — 710711712 — 719720 — 799
800 — 810811812 — 819820 — 899
900 — 910911912 — 919920 — 999
152 area codes
8 special service codes		640 CO codes
Main article: All-number calling

The original plan of 1947 had been projected to be usable beyond the year 2000. However, by the late 1950s it became apparent that it would be outgrown by about 1975.[30] The limitations for the usable leading digits of central office code, imposed by using common names for central office names, and their leading two characters as guides for customer dialing could no longer be maintained when opening new central offices. By 1962 it was forecast that in 1985 the number of telephones in the nation would equal its population of 280 million and increase to 600 million telephones for 340 million people in 2000.[29] As a result, the North American telephone administrations first introduced letter combinations that could not be linked to a familiar pronounceable central office name. Finally, they sought the removal of the memorable central office names and the introduction of all-number calling (ANC).

Under all-number calling, the number of central office prefixes increased from 540 to potentially 800, but the first two digits of the central office code were still restricted to the range 2 to 9, and the eight combinations that ended in 11 were reserved as special calling codes.[29] This increased the numbering pool for central office codes to 640, and resulted in the partitioning of the prefix space (000—999) according to the table at the right.[31]

Interchangeable central office codes

As the numbering plan grew in the 1960s under all-number calling, plan administrators at AT&T identified that by c. 1973 some of the largest area codes in urban centers might run out of central office prefixes to install more individual access lines. For relief in these cases, they finally removed the requirement that the middle digit of the central office code could not be 0 or 1. This resulted in the format of interchangeable central office codes, N X X, where N=2–9, and X is any digit. The first cities that required this action in 1974, were the city of Los Angeles with area code 213, and New York City (212). This change also required modification of the local dialing procedures to distinguish local call from long-distance calls with area code.

Requiring 1 to be dialed before the full number in some areas provided for area codes of the form N10, such as 210 in the San Antonio, Texas, area and 410 in eastern Maryland. Therefore, someone calling from San Jose, California, to Los Angeles before the change would have dialed 213-555-0123 and after the change 1-213-555-0123, which permitted the use of 213 as an exchange prefix in the San Jose area. The preceding 1 also ideally indicates a toll call; however, this is inconsistent across the NANP because the FCC has left it to the U.S. state public utilities commissions to regulate for traditional landlines, and it has since become moot for mobile phones and digital VoIP services that now offer nationwide calling without the extra digit.

Interchangeable NPA codes

In 1995, the North American Numbering Plan Administrator removed the requirement that the middle digit of an area code had to be either 0 or 1, implementing fully interchangeable NPA and central office codes, that had already been anticipated since the 1960s, when interchangeable central office codes were sanctioned.

Modern plan

The NANP numbering format may be summarized in the notation NPA-NXX-XXXX:

Component Name Number ranges Notes
NPA Numbering plan area code Allowed ranges: [2–9] for the first digit, and [0-9] for the second and third digits. When the second and third digits of an area code are the same, that code is called an easily recognizable code (ERC). ERCs designate special services; e.g., 888 for toll-free service. The NANP is not assigning area codes with 9 as the second digit.[32] Covers Canada, the United States, parts of the Caribbean Sea, and some Atlantic and Pacific islands. The area code is often enclosed in parentheses.
NXX Central office (exchange) code Allowed ranges: [2–9] for the first digit, and [0–9] for both the second and third digits (however, in geographic area codes the third digit of the exchange cannot be 1 if the second digit is also 1). Central office code: Within the NPA a uniquely assigned three-digit code.
XXXX Line number [0–9] for each of the four digits. Within a central office, a unique four-digit number, also called station code.

For example, 234-235-5678 is a valid telephone number with area code 234, central office prefix (exchange) 235, and line number 5678. The number 234-911-5678 is invalid, because the central office code must not be in the form N11. 314-159-2653 is invalid, because the office code must not begin with 1. 123-234-5678 is invalid, because the NPA must not begin with 0 or 1.

Each three-digit area code has a capacity of 7,919,900 telephone numbers.

  • Digit N of NXX is restricted to the digits 2 through 9
  • To avoid confusion with the N11 codes, the last two digits of NXX cannot both be 1.
  • Despite the widespread use of fictional telephone numbers of the form NPA 555-XXXX, only the block of line numbers from 0100 through 0199 are specifically reserved for this purpose, leaving the rest unavailable for assignment.
  • In many numbering plan areas, several NXX central office prefixes are generally not assigned: those identical to the home area code(s), adjacent domestic area codes, area codes reserved for future relief nearby, industry testing codes (generally 958 and 959) and special service codes (such as 950 and 976).

Using 0 or 1 as the first digit of an area code or seven-digit local number is invalid, as is a 9 as the middle digit of an area code; these are trunk prefixes or reserved for North American Numbering Plan expansion.

The country calling code for all countries participating in the NANP is 1. In international format, an NANP telephone number is listed as +19995550100, where 999 stands in for the area code.

International dialing

While the national numbering plan of the NANP was designed as a 10-digit closed plan, international direct distance dialing (IDDD) was accomplished by extensive modifications in switching systems to accommodate an open international numbering plan for telephone numbers from seven to twelve digits.[33]

Expansion

Canada and the United States have experienced rapid growth in the number of area codes, particularly between 1990 and 2005. The widespread adoption of fax, modem, and mobile phone communication, as well as the deregulation of local telecommunication services in the United States in the mid-1990s, increased the demand for telephone numbers.

The Federal Communications Commission allowed telecommunication companies to compete with the incumbent local exchange carriers for services, usually by forcing the existing sole service provider to lease infrastructure to other local providers. Because of the original design of the numbering plan and the telephone switching network that assumed only a single provider, number allocations had to be made in 10,000-number blocks even when much fewer numbers were required for each new vendor. Due to the proliferation of service providers in some numbering plan areas, many area codes fell into jeopardy, facing exhaustion of numbering resources. The number blocks of failed service providers often remained unused, as no regulatory mechanism existed to reclaim and reassign these numbers.

Area codes are added by two principal methods, number plan area splits and overlay plans. Splits were implemented by dividing an area into two or more regions, one of which retained the existing area code and the other areas receiving a new code. In an overlay, multiple codes are assigned to the same geographical area, obviating the need for renumbering of existing services. Subtle variations of these techniques have been used as well, such as dedicated overlays, in which the new code is reserved for a particular type of service, such as cellular phones and pagers, and concentrated overlays, in which a part of the area retained a single code while the rest of the region received an overlay code. The only service-specific overlay in the NANP was area code 917 (New York City) when it was first installed; such service-specific area code assignments were later prohibited by the Federal Communications Commission.

Most area codes of the form N10, originally reserved for AT&T's Teletypewriter eXchange (TWX) service, were transferred to Western Union in 1969 and were freed up for other use in 1981 after conversion to Telex II service was complete. The last of these, 610, was assigned to Canada, but reassigned in 1992. These new area codes, as well as a few other codes used for routing calls to Mexico, were used for telephone area code splits in the late 1980s and early 1990s, as all other area codes under the original plan had been consumed.

After the remaining valid area codes were used up by expansion, in 1995 the rapid increase in the need for more area codes forced the NANPA to allow the digits 2 through 8 to be used as a middle digit in new area code assignments, with 9 being reserved as a last resort for potential future expansion. At the same time, local exchanges were allowed to use 1 or 0 as a middle digit. The first area codes without a 1 or 0 as the middle digit were area code 334 in Alabama and area code 360 in Washington, which both began service on January 15, 1995. This was quickly followed by area code 520 serving Arizona on March 19, 1995.

Codes ending in double digits are reserved as easily recognizable codes (ERCs), to be used for special purposes such as toll-free numbers, personal 500 numbers, Canadian non-geographic area code 600, carrier-specific 700 numbers, and high-toll 900 numbers, rather than for geographic areas. Nevada was denied 777 ("lucky 7s", a reference to the state's legalized gambling) for this reason;[34] it received 775 instead when most of Nevada split from 702, which continues to serve the Las Vegas metropolitan area.

By 1995, many cities in the United States and Canada had more than one area code, either from dividing a city into different areas (NPA split) or having more than one code for the same area (NPA overlay). The overlay method requires that the area code must be dialed in all cases, even for local calls, while the split plan may permit seven-digit dialing within the same area. The transition to ten-digit dialing typically starts with a permissive dialing phase, which is widely publicized, during which dialing all ten digits is optional. After a period of several months, mandatory dialing begins, when seven-digit dialing is no longer permissible. Atlanta was the first U.S. city to require mandatory ten-digit dialing throughout the metropolitan area, roughly coinciding with the 1996 Summer Olympics held there. Atlanta was used as the test case not only because of its size, but also because it had the world's largest fiber optic network at the time, five times larger than that of New York, and it was home to BellSouth (now part of AT&T), then the Southeastern Regional Bell Operating Company, with AT&T's fiber optics manufacturing facility within the city.

Common dialing methods
7-digit dialingNXX xxxxNPA code not required
10-digit dialingNPA NXX xxxx
11-digit dialing1 NPA NXX xxxx1 is the NANP trunk prefix for long-distance circuits

Expansion issues

Depending on the techniques used for area code expansion, the effect on telephone users varies. In areas in which overlays were used, this generally avoids the need for converting telephone numbers, so existing directories, business records, letterheads, business cards, advertising, and "speed-dialing" settings can retain the same phone numbers, while the overlay is used for new number allocations. The primary impact on telephone users is the necessity of remembering and dialing 10- or 11-digit numbers when only 7-digit dialing was previously permissible.

Splitting instead of overlaying generally avoids the requirement for mandatory area-code dialing, but at the expense of having to convert a region to the new code. In addition to the requirements of updating records and directories to accommodate the new numbers, for efficient conversion this requires a period of "permissive dialing" in which the new and old codes are both allowed to work. Also, many splittings involved significant technical issues, especially when the area splittings occurred over boundaries other than phone network divisions.

In 1998 area code 612, which had covered the Minneapolis – Saint Paul Twin Cities, was split to create area code 651 for St. Paul and the eastern metropolitan area. The Minnesota Public Utilities Commission mandated that the new boundary exactly follow municipal boundaries, which were distinctly different from telephone exchange boundaries, and that all subscribers keep their 7-digit numbers. These two goals were directly at odds with the reason for the split, namely to provide additional phone numbers. More than 40 exchanges had territory that straddled the new boundary. As a result, prefixes were duplicated in both area codes, which counteracted much of the benefit of the splitting, with only 200 of 700 prefixes in area 612 moving entirely to area 651. In less than two years area code 612 again exhausted its supply of phone numbers, and required a three-way split in 2000, creating the new area codes 763 and 952. Again, the split followed political boundaries rather than rate center boundaries, resulting in additional split prefixes; a few numbers moved from 612 to 651 and then to 763 in less than two years.

Decrease in expansion rate

Recognizing that the proliferation of area codes was largely due to the telecom regulation act and the assignment of numbers in blocks of 10,000, the FCC instructed NANPA, by then administered by Neustar, to alleviate the numbering shortage. As a result, number pooling was piloted in 2001 as a system for allocating local numbers to carriers in blocks of 1,000 rather than 10,000. Because of the then design of the switched telephone network, this was a considerable technical obstacle. Number pooling was implemented with another technical obstacle, local number portability.

The program has been implemented in much of the United States by state regulators. A limited number of cities have also implemented rate center consolidation; fewer rate centers resulted in more efficient use of numbers, as carriers would reserve blocks of 1,000 or 10,000 numbers in each of multiple rate centers in the same area even if they had relatively few clients in the area.[35] (A rate center is a geographical area used by a Local Exchange Carrier (LEC) to determine the boundaries for local calling, billing and assigning phone numbers. Typically a call within a rate center is local, while a call from one rate center to another is a long-distance call.) Together with aggressive reclamation of unused number blocks from telecom providers, number pooling has reduced the need for additional area codes, so that many previously designated area splits and overlays have been postponed indefinitely.

There is no number pooling in Canada. Number allocation remains highly inefficient as even the tiniest village is a rate center and every CLEC is assigned blocks of ten thousand numbers in every place it offers new local service. As a result, dialing seven digits even in remote locations like James Bay is more likely to produce an intercept message ("dial the area code") than an actual voice connection.

New area codes outside the contiguous United States and Canada

Before 1995, all NANP countries and territories outside the contiguous United States, Alaska, Hawaii and Canada shared the area code 809. This included Puerto Rico and the U.S. Virgin Islands. Each has since been assigned one or more distinct numbering plan areas; area code 809 now exclusively serves the Dominican Republic (along with area codes 829 and 849). The United States Pacific territories of the Northern Mariana Islands and Guam joined the NANP in 1997, and American Samoa became an NANP member in October 2004. The Dutch possession of Sint Maarten was originally scheduled to join the NANP on May 31, 2010, but the changeover was postponed to September 30, 2011.[14]

Bermudabefore 1995: served by area code 8091995: assigned area code 441
Puerto Ricobefore 1996: served by area code 8091996: assigned area code 787

2001: overlaid with area code 939

U.S. Virgin Islandsbefore 1997: served by area code 8091997: assigned area code 340
Northern Marianasbefore 1997: reached via IDDD using country code 6701997: assigned area code 670
Guambefore 1997: reached via IDDD using country code 6711997: assigned area code 671
American Samoabefore Oct. 1, 2004: reached via IDDD using country code 6842004: assigned area code 684
Sint Maartenbefore Sept. 30, 2011: reached via IDDD using country code 5992011: assigned area code 721

Number size expansion
Main article: North American Numbering Plan expansion

The NANP exhaust analysis estimates that the existing numbering system is sufficient beyond 2049, based on the assumptions that a maximum of 674 NPAs continue to be available, and that on average 3990 central office codes are needed per year.[36]

In case of exhaustion, various plans are discussed for expanding the numbering plan. One option is to add the digit 1 or 0 either at the beginning or at the end of the area code, or prefixing it to the seven-digit subscriber number. This would require eleven-digit dialing even for local calls between any two NANP numbers. Another proposal introduces the digit 9 into the area code in the format x9xx, so that, for example, San Francisco's 415 would become 4915. Other proposals include reallocating blocks of numbers assigned to smaller long-distance carriers or unused reserved services.

Countries and territories
Countries participating in the North American Numbering Plan

Of all states or territories, the U.S. state of California has the largest number of area codes assigned, followed by Texas, Florida and New York, while most countries of the Caribbean use only one.[37] Many Caribbean codes were assigned based on alphabetic abbreviations of the territory name, as indicated in the third column of the following table (Letter code). This follows the traditional letter assignments on telephone dials. For some Pacific islands, the NANPA area code is the same as the country code that was discontinued upon membership in the NANP.

Country/TerritoryArea codesLetter code
 American Samoa684*
 Anguilla264ANG
 Antigua and Barbuda268ANT
 Bahamas242BHA
 Barbados246BIM
 Bermuda441
 British Virgin Islands284BVI
Canada204, 226, ... 905
 Cayman Islands345
 Dominica767ROS (Roseau)
 Dominican Republic809, 829, 849
 Grenada473GRE
 Guam671*
 Jamaica876, 658[38]
 Montserrat664
 Northern Mariana Islands670*
 Puerto Rico787, 939PUR (787)
 Saint Kitts and Nevis869
 Saint Lucia758SLU
 Saint Vincent and the Grenadines784SVG
 Sint Maarten[14]721
 Trinidad and Tobago868TNT
 Turks and Caicos Islands649
 United States201, 202, ... 989
 United States Virgin Islands340
* same as previous country code

Membership in the NANP brings significant advantages for countries in the vicinity of the United States and Canada, which usually are already the top dialing destinations. Both countries also originate most of the tourism business for the Caribbean. This is enhanced by the integration from sharing the same dialing procedures, without international access codes, and the toll-free number system of the NANP, as businesses in all member countries are eligible to participate.

Dialing procedures

The structure of the North American Numbering Plan permits implementation of local dial plans in each plan area, depending on requirements. When multiple NPA codes serve an area in an overlay arrangement, ten-digit (10D) dialing is required. Seven-digit (7D) dialing may be permissible in areas with single area codes.[39] Depending on the requirement of toll alerting, it may be necessary to prefix a telephone number with 1. The NANPA publishes dial plan information for individual area codes.[40]

The standard dial plans in most cases are as follows:

Local within area code Local outside area code Toll within area code Toll outside area code
Single code area, with toll alerting 7D 7D or 10D 1+10D 1+10D
Single code area, without toll alerting 7D 1+10D 7D or 1+10D 1+10D
Overlaid area, with toll alerting 10D 10D 1+10D 1+10D
Overlaid area, without toll alerting 10D or 1+10D 1+10D 10D or 1+10D 1+10D

The number of digits dialed is unrelated to being a local call or a toll call when there is no toll alerting. Allowing 7D local dial across an area code boundary, which is uncommon today, requires central office code protection, locally if using toll alerting, across the entire area code otherwise, to avoid assignment of the same seven-digit number on both sides.

Most areas permit local calls as 1+10D except for Texas, Georgia, and some jurisdictions in Canada which require that landline callers know which numbers are local and which are toll, dialing 10D for local calls and 1+10D for all toll calls.

In almost all cases, domestic operator-assisted calls are dialed 0+10D.

Special numbers and codes
See also: N11 code

Some common special numbers in the North American system:

  • 0 – Operator assistance
  • 00 – Long-distance operator assistance (formerly 2-1-1)
  • 011 – International access code using direct dial (for all destinations outside the NANP).
  • 01 – International access code using operator assistance (for all destinations outside the NANP).
  • 101-xxxx – Used to select use of an alternative long-distance carrier
  • 211 – Local community information or social services (in some cities)
  • 311 – City government or non-emergency police matters
  • 411 – Local telephone directory service (Some telephone companies provide national directory assistance)
  • 511 – Traffic, road, and tourist information
  • 611 – Telephone line repair service (formerly 4104), wireless operator customer service (formerly 811).
  • 711 – Relay service for customers with hearing or speech disabilities.
  • 811 – Dig safe pipe/cable location in the United States, non-urgent telehealth/teletriage services in Canada (formerly telephone company business office)
  • 911 – Emergency telephone number – fire department, medical emergency, police.
  • 950-xxxx – Feature group code for access to a carrier from a non-subscriber location. The feature requires the customer dial a 950-xxxx number and enter a calling card number and destination telephone number. It was originally used for locations where 101-xxxx dialing was not possible.
  • 958-xxxx (local); 959-xxxx (long distance) – Plant test numbers, such as automatic number announcement circuits. It was once common to reserve entire unused exchange prefixes or N11 numbers (4101 was ringback number on many step-by-step switches), but these have largely moved to individual unpublished numbers within the standard 958-xxxx (local) or 959-xxxx (long-distance) plant test exchanges as numbers become scarce.
  • 1-NPA-555-1212 – Non-local directory information (Canada and United States)
  • In December 2019, the Federal Communications Commission proposed making 9-8-8 a national number in the United States for the National Suicide Prevention Lifeline.[41]

There are also special codes, such as:

  • *51 and 1151: A history of unanswered calls on a telephone number, useful for those who are not Caller ID subscribers.
  • *57 and 1157: Used to trace harassing, threatening, abusive, obscene, etc. phone calls, and keep results of trace at phone company.
  • *66 and 1166: To keep retrying a busy-line (see also Called-party camp-on)
  • *67 and 1167: Caller ID Block
  • *69 and 1169: Call Return caller may press '1' to return call after hearing number
  • *70 and 1170: Cancel call waiting on a call-by-call basis
  • *71 and 1171: Three-way calling, which lets a person talk to people in two different locations at the same time.
  • *74 and 1174: Speed dial, which allows someone to quickly dial any of eight frequently called numbers using a one-digit code, from any phone on their line.
  • *75 allows a total of 30 speed-call numbers with two digits.
  • *77 activates Anonymous Call Rejection Service
  • *82 and 1182: Releases Caller ID block on a call-by-call basis
  • *87 deactivates Anonymous Call Rejection Service

Note: The four-digit numbers are not implemented in some areas. The codes prefixed with an asterisk (*) symbol are intended for use on Touch-Tone telephones, whereas the four-digit numbers prefixed 11xx are intended for use on rotary dial telephones, where the Touch-Tone * symbol is not available. Not all NANP countries use the same codes. For example, the emergency telephone number is not always 911: Trinidad and Tobago and Dominica use 999, as in the United Kingdom. The country of Barbados uses 211 for police force, 311 for fire, and 511 for ambulance, while Jamaica uses 114 for directory assistance, 119 for police force, and 110 for fire and ambulance services.

Despite its early importance as a share of the worldwide telephone system, few of the NANP's codes, such as 911, have been adopted outside the system. Determining that 911 requires unnecessary rotation time on rotary dial telephones, the European Union has adopted its own standardized number of 112, while countries in Asia and the rest of the world use a variety of other two- or three-digit emergency telephone number combinations. The 112 code is gaining prevalence because of its preprogrammed presence in mobile telephones that conform to the European GSM standard. The European Union and many other countries have chosen the International Telecommunication Union's 00 as their international access number instead of 011. The toll-free prefix 800 has been widely adopted elsewhere, including as the international toll-free country code. It is often preceded by a 0 rather than a 1 in many countries where 0 is the trunk prefix.

Alphabetic mnemonic system
digitletters
Letters of the alphabet are mapped to the digits of the telephone dial pad.
2ABC
3DEF
4GHI
5JKL
6MNO
7P(Q)RS
8TUV
9WXY(Z)

Many dials on modern telephones in use in the NANP service areas maintain the tradition of alphabetic dialing. Usually each pushbutton from digit 2 to 9 also displays three letters, which is standardized in ISO 9995-8 and, in Europe, E.161. Historically, the letters Q and Z were omitted, although some modern telephones contain them. SMS-capable devices have all 26 letters. The alphabet is apportioned to the buttons as follows:

No letters are typically mapped to keys 1 and 0, although some corporate voicemail systems use 1 for Q and Z, and some old telephones assigned the Z to the digit 0.

Originally, this scheme was used as a mnemonic device for telephone number prefixes. When telephone numbers in the United States were standardized in the mid-20th century to seven digits, the first two digits of the exchange prefix were expressed as letters rather than numbers, using the telephone exchange name. Before World War II, the largest cities used three letters and four or five numbers, while in most cities with customer dialing, phone numbers had only six digits (2L-4N). The prefix was a name, and the first two or three letters, usually shown in capital letters, were dialed. Later, the third letter, where implemented, was replaced by a digit, or an extra digit was added. This generally happened after World War II, although New York City converted in 1930. The adoption of seven-digit local numbers (2L-5N) was chosen as the requirement for direct distance dialing and progressively deployed starting the late 1940s.

The famous Glenn Miller tune PEnnsylvania 6-5000 refers to telephone number PE6-5000, a number still in service at the Hotel Pennsylvania (212 736-5000) in New York. Similarly, the classic film BUtterfield 8 is set in the East Side of Manhattan between roughly 64th and 86th Streets, where the telephone prefixes include 288. In some works of fiction, phone numbers will begin with "KLondike 5" or KLamath 5, which translates to 555, an exchange that is reserved for information numbers in North America.

The letter system was phased out, beginning before 1965, although it persisted ten years later in some places. It was included in Bell of Pennsylvania directories until 1983. Even today, some businesses still display a 2L-5N number in advertisements, e.g., the Belvedere Construction Company in Detroit, Michigan not only still uses the 2L-5N format for its number (TYler 8-7100), it uses the format for the toll-free number (1-800-TY8-7100).

Despite the phasing out of the letter system otherwise, alphabetic phonewords remain as a commercial mnemonic gimmick, particularly for toll-free numbers. For example, one can dial 1-800-FLOWERS to order flowers, or 1-800-DENTIST to find a local dentist.

In addition to commercial uses, alphabetic dialing has occasionally influenced the choice of regional area codes in the United States. For example, when area 423 (East Tennessee) was split in 1999, the region surrounding Knoxville was assigned area code 865, chosen to represent the word VOL (Volunteers), the nickname of Tennessee (The Volunteer State), as well as athletic teams at the University of Tennessee.[42][43] Another example of this is area code 859 in Kentucky, which was chosen to represent 'UKY' as a nod to the University of Kentucky in Lexington, Kentucky, the code's principal city.

Several Caribbean area codes were chosen as an alphabetic abbreviation of the country name, which are indicated in the table of NANP regions.

Cellular services

The North American Numbering Plan does not set aside special non-geographic area codes exclusively for cellular phones. Only one regional exception exists in area code 600 in Canada. In many other national numbering plans outside the NANP, mobile services are assigned separate prefixes. Cell phone numbers in the NANP are allocated within each area code from special central office prefixes and calls to them are billed at the same rate as any other call. Consequently, the caller pays pricing model adopted in other countries, in which calls to cellular phones are charged at a higher nationwide rate, but incoming mobile calls are not charged to the mobile user, could not be implemented. Instead, North American cellular telephone subscribers are also generally charged for receiving calls (subscriber pays). This has discouraged mobile users from publishing their telephone number. However, price competition among carriers has reduced the average price per call minute for contract customers for both inbound and outbound calls, which compare favorably to those in caller-pays countries. Most users select bundle pricing plans that include an allotment of minutes expected to be used in the billing period. Of the four major national carriers in the United States, all four (AT&T, T-Mobile/Sprint, Verizon) offer free calling between mobile phones on the carrier's network, and Sprint also offers its customers free calling to mobile phones on other networks.

Industry observers have attributed the relatively low mobile phone penetration rate in the United States, compared to that of Europe, to the subscriber-pays model. In this model the convenience of the mobility is charged to the subscriber. Callers from outside the local-calling region of the assigned number, however, pay for a long-distance call, although domestic long-distance rates are generally lower than the rates in caller-pays systems. Conversely, an advantage of caller-pays is the relative absence of telemarketing and nuisance calls to mobile numbers. The integrated numbering plan also enables local number portability between fixed and wireless services within a region, allowing users to switch to mobile service while keeping their telephone number.

The initial plan for area code overlays did allow for providing separate area codes for use by mobile devices, although these were still assigned to a specific geographical area, and were charged at the same rate as other area codes. Initially, the area code 917 for New York City was specifically assigned for this purpose within the boroughs; however, a Federal court overturned the practice and the use of an area code for a specific telephony purpose. Since mobile telephony has been expanding faster than landline use, new area codes typically have a disproportionately large fraction of mobile and nomadic numbers, although landline and other services rapidly follow and local network portability can blur these distinctions.

The experience of Hurricane Katrina and similar events revealed a possible disadvantage of the methods employed in the geographic assignment of cellular numbers. Many mobile phone users could not be reached, even when they were far from the stricken areas, because the routing of calls to their phones depended on equipment in the affected area. They could make calls but not receive them.[44]

The use of geographic numbers may also lead to tromboning; one can take a handset with a Vancouver number into St. John's and outbound calls to St. John's numbers while in that city will be local, but incoming calls must make the cross-country trip to Vancouver and back. This adds costs for subscribers, as an 8,000 km cross-country call (as a worst case) incurs long-distance tolls in both directions. AMPS subscribers used to be provided with a local number (such as 1-NPA-NXX-ROAM) in each city, allowing them to be reached by dialing that number plus the ten-digit mobile telephone number; this is no longer supported.

Toll charges

Calls between different countries and territories of the NANP are not typically charged at domestic rates. For example, most long-distance plans may charge a California subscriber a higher rate for a call to British Columbia than for a call to New York, even though both destinations are within the NANP. Similarly, calls from Bermuda to U.S. numbers (including 1-800 numbers, which are normally thought of as toll-free) incur international rates. This is because many of the island nations implemented a plan of subsidizing the cost of local phone services by directly charging higher pricing levies on international long-distance services.

Because of these higher fees, scams had taken advantage of customers' unfamiliarity with pricing structure to call the legacy regional area code 809. Some scams lured customers from the United States and Canada into placing expensive calls to the Caribbean, by representing area code 809 as a regular domestic, low-cost, or toll-free call. The split of 809 (which formerly covered all of the Caribbean NANP points) into multiple new area codes created many new, unfamiliar prefixes which could be mistaken for U.S. or Canada domestic area codes but carried high tariffs. In various island nations, premium exchanges such as +1-876-HOT-, +1-876-WET- or +1-876-SEX- (where 876 is Jamaica) became a means to circumvent consumer-protection laws governing area code 900 or similar U.S.-domestic premium numbers.

These scams are on the decline, with many of the Cable and Wireless service monopolies being opened up to competition, hence bringing rates down. Additionally, many Caribbean territories have implemented local government agencies to regulate telecommunications rates of providers.[45][46]

Number portability

The Telecommunications Act of 1996 (47 U.S.C. § 251 (b)(2)) authorizes the Federal Communications Commission (FCC) to require all local exchange carriers (LECs) to offer local number portability.[47] The FCC regulations were enacted on June 27, 1996, with changes to take effect in the one hundred largest Metropolitan Statistical Areas by October 1, 1997 and elsewhere by December 31, 1998.[48]

The FCC directed the North American Numbering Council (NANC) to select one or more private-sector candidates for the local number portability administrator (LNPA) function,[49] in a manner akin to the selection of the North American Numbering Plan Administrator (NANPA).[50]

The toll-free telephone numbers in NPA 800, 888, 877, 866, 855, and 844 have been portable through the RespOrg system since 1993.[51]

Fictional telephone numbers

American television programs and films often use the central office code 555, or KLamath 5 and KLondike 5 in older movies and shows, for fictitious telephone numbers, to prevent disturbing actual telephone subscribers if anyone is tempted to dial a telephone number seen or referred to on screen.

Occasionally, valid telephone numbers are used in contexts such as songs with varying intents and consequences. An example is the 1981 song "867-5309/Jenny" by Tommy Tutone, which is the cause of a large number of calls.[52]

Not all numbers beginning with 555 are fictional. For example, 555-1212 is the standard number for directory assistance. Only 555-0100 through 555-0199 are reserved for fictional use. Where used, these are often routed to information services; Canadian telephone companies briefly promoted 555-1313 as a pay-per-use "name that number" reverse lookup in the mid-1990s.[53]

See also

References
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  2. "Canadian Numbering Administrator".
  3. "NANPA : Number Resources - NPA (Area) Codes". Retrieved 2015-10-23.
  4. ITU; ITU-T. "The International Public Telecommunication Numbering Plan". ITU. Retrieved 2015-07-25.
  5. Nunn, W. H. (1952). "Nationwide Numbering Plan". Bell System Technical Journal. 31 (5): 851–9. doi:10.1002/j.1538-7305.1952.tb01412.x.
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  10. Mehta, Stephanie N. (1999-02-26). "The Kennedy Space Center Acquires A New Area Code: 3-2-1, as in Blast Off". The Wall Street Journal.
  11. AT&T, Notes on the Network, Section 3, p.8 (1980)
  12. Green book, Volume 2, Part 1, International Telegraph and Telephone Consultative Committee, International Telecommunication Union, 1973, page 129
  13. "Phone System Changes Kill 2 of Mexico's Area Codes - latimes". 2015-10-18. Archived from the original on 2015-10-18.CS1 maint: bot: original URL status unknown (link)
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  16. Bellcore Planning Letter PL-NANP-106, (1997-12-23)
  17. "About NANPA". Neustar. Retrieved 2015-06-23.
  18. [https://docs.fcc.gov/public/attachments/DOC-354567A1.pdf FCC News release: FCC selects Somos as NANPA and PA under one-year bridge contract.
  19. https://www.somos.com/insights/somos-inc-awarded-north-american-numbering-plan-administration-and-pooling-administration-contracts
  20. AT&T (1955) Notes on Nationwide Dialing
  21. Notes on the Network, AT&T (1980)
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  23. AT&T, Notes on the Network, Section 10, p.3 (1980).
  24. John Greene (2015-01-16). "Why Did Old Phone Numbers Start With Letters?". Mental Floss. Retrieved 2019-02-01.
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  26. "Now You Can Call, If Your Calls Don't Work Some Business Lines Aren't Set Up To Call To New Area Codes" Archived 2008-03-16 at the Wayback Machine, The Virginian-Pilot, November 1, 1995. Accessed June 8, 2007. "When the first area code, 201, was introduced in New Jersey in 1951, phone-numbering experts thought there would be enough codes with a middle digit of 0 or 1 to last well into the next century."
  27. Statistical Abstract of the United States. 1960. p. 516. Retrieved 2019-02-01.
  28. Bell Telephone Laboratories, A History of Engineering and Science in the Bell System - The Early Years (1875-1925), M.D. Fagan (ed.), 1975, p.126
  29. Blair N.D., Cosgrove M.P. (AT&T), why all numbers?, Bell Telephone Magazine, Autumn 1962, p.10
  30. AT&T, All-Number Calling Being Introduced In Bell System, Bell Laboratories Record 38(12) p.470 (December 1960)
  31. Bell Telephone Laboratories, Engineering and Operations in the Bell System (1984), p.119
  32. "NANPA : Number Resources - NPA (Area) Codes".
  33. AT&T, Notes on the Network, Section 10.3.02, p.3 (1980).
  34. "Additional area code being considered for Las Vegas". 2012-07-19.
  35. Rate Center Consolidations
  36. "April 2019 North American Numbering Plan (NANP) Exhaust Analysis" (PDF).
  37. "United States Area Codes". www.areacodehelp.com. Retrieved 2018-09-27.
  38. "Call 658 ... Jamaica gets additional area code, 10-digit dialling becomes mandatory May 2018".
  39. "Uniform Dialing Plans".
  40. "Area Codes Requiring 10 Digit Dialing".
  41. FCC unanimously approves proposal for new 3-digit number as Suicide Prevention Hotline
  42. Brewer, Bill. 423 Area Code To Become VOL In 9 ET Counties. Knoxville News-Sentinel. April 17, 1999.
  43. Tennessee Regulatory Authority press release, April 29, 1999
  44. NBC News, “Katrina outages reveal phone system quirks”, retrieved 14 Feb 2018.
  45. Administrator. "Fair Trading Commission, Barbados - Legislation".
  46. "The Eastern Caribbean Telecommunications Authority (ECTEL) > Telecom regulations". Archived from the original on 2011-07-18. Retrieved 2009-03-13.
  47. 11 FCC Rcd 8353
  48. 11 FCC Rcd 8355. The regulations are located at 47 CFR 52, 47 CFR 52.20 et seq.
  49. 47 CFR 1.1204
  50. 11 FCC Rcd 8401
  51. 10 FCC Rcd 12351
  52. "867-5309/Jenny". Urban Legends Reference Pages. snopes.com. 2014-07-09. Retrieved 2017-06-28.
  53. "Canadian telco offers users a handy reverse directory". America's Network. 1996-05-15.
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