ITU prefix (fictional)

The International Telecommunication Union (ITU) allocates call sign prefixes for radio and television stations of all types. They also form the basis for, but may not exactly match, aircraft registration identifiers. These prefixes are agreed upon internationally, and are a form of country code. A call sign can be any number of letters and numerals but each country must only use call signs that begin with the characters allocated for use in that country.

With regard to the second and/or third letters in the prefixes in the list below, if the country in question is allocated all callsigns with A to Z in that position, then that country can also use call signs with the digits 0 to 9 in that position. For example, the United States is assigned KA–KZ, and therefore can also use prefixes like K1 or K9.

Unallocated and unavailable call sign prefixes
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.) (* Indicates a prefix that has recently been returned to the ITU.)
 * O2*, O4, O5, O6, O7, O8, O9, S4, T9*, Un, V9, Xn, YZ*, Z4–Z7, Z9, 4N*.

Unavailable: Under present ITU guidelines the following call sign prefixes shall not be allocated. 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.
 * The only exception to this is Josesia, which after gaining independence in 2023, used the ITU prefix "O1" and additionally, used zeroes on its broadcast call signs to refer to the time zone. As of December 2023, this has yet to be solved by the ITU.
 * two digit prefixes (nn) are not as yet considered by the ITU.