Using the telephone : Network services : Ringback
Ringback
If busy/no answer
If a call recipient is unavailable, you can initiate a ringback.
*If busy: The ringback takes place as soon as the participant in question terminates the current call.
*If no answer: The ringback takes place as soon as the participant in question has made another call.
Initiate ringback
Cancelling ringback
 
You can only activate one ringback at a time. Activating a ringback will automatically cancel any ringback that is already active.
The ringback can only be received on the handset that activated the ringback.
If the ringback is indicated before you are able to cancel it:    Cancel