ICAO Annex 10
Volume II - Chapter 5
5.2.1.7.2 Radiotelephony call signs for aircraft
5.2.1.7.2.1 Full call signs

5.2.1.7.2.1.1 An aircraft radiotelephony call sign shall be one of the following types:

Type a) The characters corresponding to the registration marking of the aircraft; or
Type b) The telephony designator of the aircraft operating agency, followed by the last four characters of the registration marking of the aircraft;

Type c) The telephony designator of the aircraft operating agency, followed by the flight identification.

5.2.1.7.2.2 Abbreviated call signs
5.2.1.7.2.2.1 The aircraft radiotelephony call signs shown in 5.2.1.7.2.1.1, with the exception of Type c), may be abbreviated in the circumstances prescribed in 5.2.1.7.3.3.1. Abbreviated call signs shall be in the following form:

Type a) - the first character of the registration and at least the last two characters of the call sign;
Type b) - the telephony designator of the aircraft operating agency, followed by at least the last two characters of the call sign;

Type c) - no abbreviated form.

Table 5-1 Examples of full call signs and abbreviated call signs
(see 5.2.1.7.2.1 and 5.2.1.7.2.2)