Automatic ring back
Automatic ring back is a service offered by phone companies.
Normally, when a person's line is busy, one has to call back every few minutes to check if their line is free yet. With automatic ring back, a code can be dialed into the telephone keypad to enable ringback. Then, when their line is free, the original caller's phone rings with a distinctive ring so that one knows it is automatic ring back and not a regular call. When the phone is picked up, it calls the other number since the line is now free.
In the UK the service is also known as Ringback.