AIC 69/2005
4 Telephony Designators
4.1 With the exception of State aircraft, ICAO Telephony Designators will normally only be assigned to AOC holders. Normally, no more than one Telephony Designator will be registered for each aircraft operating agency, aeronautical authority or service.
4.2 Exceptionally, Telephony Designators may be assigned to non-AOC holders at the discretion of DAP. In such cases, the assigned Telephony Designator may be used for flights operated in accordance with aircraft and pilot licensing requirements. Telephony Designators will not under any circumstances be assigned to individuals for personal use.
4.3 Telephony Designators may be used as part of the radiotelephony callsign in accordance with ICAO Radiotelephony Procedures (Annex 10, Volume 2) and CAP 413 (Radiotelephony Manual). Telephony Designators should resemble as far as possible the name of the aircraft operating agency or its function. Ideally, there should be a correlation between the three-letter designator, the Telephony Designator and the name of the aircraft operating agency or its function.
4.4 In order to reduce the length of R/T transmissions, Telephony Designators should be brief and comprise if possible one word of two or three syllables. It should not exceed two words. The Telephony Designator should be easily and phonetically pronounceable.
4.5 The use by aircraft operators of Telephony Designators relating to an air traffic service unit is not recommended and should be avoided.