Users can request a Callback when they reach a busy or unanswered station.
Callback with service retention is supported for the following Call Completion (CC) Supplementary Services:
Completion of Calls to Busy Subscribers (SS-CCBS): users can set a Callback against a busy station.
Completion of Calls on No Reply (SS-CCNR): users can set a Callback against a station that doesn't answer.
Note: Service retention is the capability to continue with a CC Request after the CC Call failed.