The main idea behind callback is that when user A wishes to have a phone conversation with user B, he does not make an outgoing call to user B (as he would do with normal telephony service); instead:
- He initiates a callback call (we will discuss the available methods for triggering a callback later).
- The system establishes a call to A, and instead of originating the call, A actually answers it (in most telephony networks a subscriber does not pay for incoming calls, so this call is free for A).
- The system establishes a call to B and then bridges the two calls together so that A and B can communicate.
