Air traffic control may instruct you to temporarily change your call sign to prevent confusion with another aircraft, on the same frequency, and with a similar call sign. For example:

• Aircraft 1: call sign 'Bottle 323 '
• Aircraft 2: call sign 'Throttle 232 '
• So, ATC instructs Aircraft 2 to change it's call sign to 'Fueltap 232 '

Once the chance of confusion ends, for example aircraft 1 transfers to another frequency, then aircraft 2 will be instructed to resume using their original flight plan call sign.

ICAO Doc 4444
15.7.6 Change of radiotelephony call sign for aircraft

15.7.6.1 An ATC unit may instruct an aircraft to change its type of RTF call sign, in the interests of safety, when similarity between two or more aircraft RTF call signs are such that confusion is likely to occur.

15.7.6.1.1 Any such change to the type of call sign shall be temporary and shall be applicable only within the airspace(s) where the confusion is likely to occur.

15.7.6.2 To avoid confusion, the ATC unit should, if appropriate, identify the aircraft which will be instructed to change its call sign by referring to its position and/or level.

15.7.6.3 When an ATC unit changes the type of call sign of an aircraft, that unit shall ensure that the aircraft reverts to the call sign indicated by the flight plan when the aircraft is transferred to another ATC unit, except when the call sign change has been coordinated between the two ATC units concerned.

15.7.6.4 The appropriate ATC unit shall advise the aircraft concerned when it is to revert to the call sign indicated by the flight plan.