A controller may instruct an aircraft to change its type of RTF call sign, when similarity between two or more call signs are likely to cause call-sign confusion. These changes are temporary and restricted only to the airspace where the confusion is likely to occur. When this possibility no longer exists, the controller instructs the flight crew to revert to the call sign in the flight plan. This normally happens before the aircraft is transferred to another ATC unit (or sector). It is possible to transfer the aircraft using the new call sign if coordinated between the controllers of the two units (or sectors).
Due to the two similar call signs on the frequency, there is a risk that the instruction is acknowledged by the wrong aircraft. To avoid confusion, the controller identifies again the aircraft that has changed the call sign using an appropriate method (e.g. position report or an SSR code report).
The following phraseology is used in the process:
CHANGE YOUR CALL SIGN TO (new call sign) [UNTIL FURTHER ADVISED]
REVERT TO FLIGHT PLAN CALL SIGN (call sign) [AT (significant point)]