ITU prefix

Unallocated: The following call sign prefixes are available for future allocation by the ITU. (x represents any letter; n represents any digit from 2–9.)

• E8, E9, H5, J9, On, S4, T9*, Un, V9, Xn, YZ*, Z4–Z7, Z9, 4N*.

(* Indicates a prefix that has recently been returned to the ITU.)[Note 1]

Unavailable: Under present ITU guidelines the following call sign prefixes shall not be allocated.[3] They are sometimes used unofficially – such as amateur radio operators operating in a disputed territory or in a nation state that has no official prefix (e.g. S0 in Western Sahara, station 1A0 at Knights of Malta headquarters in Rome, or station 1L in Liberland). (x represents any letter; n represents any digit from 2–9.)

• nn, x0, x1, 0x, 1x, Qx.
• no prefixes beginning with Q are used—they may be confused with Q codes. Note that this applies to prefixes only - suffixes are the responsibility of the allocating country.
• no prefixes with the digits 1 or 0 are used—they may be confused with the letters I or O.
• two digit prefixes (nn) are not as yet considered by the ITU.

Linked country codes are from ISO 3166-1.

       Series allocated to an international organization.
       Provisional allocation in accordance with No. S19.33: "Between radiocommunication conferences, the Secretary-General is authorized to deal with questions relating to changes in the allocation of series of call signs, on a provisional basis, and subject to confirmation by the following conference."
       Half series allocation. The first country listed uses all callsigns beginning with the listed prefix followed by A-M, and the second country listed uses N-Z.

• Call-sign allocation plan
• Non-ITU prefix
• Amateur radio call signs
• Aircraft registration

• ITU table of international callsigns