The following is an incredibly bare-bones example sip.conf and dialplan to show basic usage of generic call completion. It is likely that if you have a more complex setup, you will need to make use of items like the CALLCOMPLETION dialplan function or the CC_INTERFACES channel variable.
First, let's establish a very simple sip.conf to use for this
Now, let's write a simple dialplan
Scenario 1: Mark picks up his phone and dials Richard by dialing 2000. Richard is currently on a call, so Mark hears a busy signal. Mark then hangs up, picks up the phone and dials 30 to call the CallCompletionRequest application. After some time, Richard finishes his call and hangs up. Mark is automatically called back by Asterisk. When Mark picks up his phone, Asterisk will dial extension 2000 for him.
Scenario 2: Richard picks up his phone and dials Mark by dialing 1000. Mark has stepped away from his desk, and so he is unable to answer the phone within the 20 second dial timeout. Richard hangs up, picks the phone back up and then dials 30 to request call completion. Mark gets back to his desk and dials somebody's number. When Mark finishes the call, Asterisk detects that Mark's phone has had some activity and has become available again and rings Richard's phone. Once Richard picks up, Asterisk automatically dials exteision 1000 for him.
Scenario 3: Much like scenario 1, Mark calls Richard and Richard is busy. Mark hangs up, picks the phone back up and then dials 30 to request call completion. After a little while, Mark realizes he doesn't actually need to talk to Richard, so he dials 31 to cancel call completion. When Richard becomes free, Mark will not automatically be redialed by Asterisk.
Scenario 4: Richard calls Mark, but Mark is busy. About thirty seconds later, Richard decides that he should perhaps request call completion. However, since Richard's phone has the default cc_offer_timer of 20 seconds, he has run out of time to request call completion. He instead must attempt to dial Mark again manually. If Mark is still busy, Richard can attempt to request call completion on this second call instead.
Scenario 5: Mark calls Richard, and Richard is busy. Mark requests call completion. Richard does not finish his current call for another 2 hours (7200 seconds). Since Mark has the default ccbs_available_timer of 4800 seconds set, Mark will not be automatically recalled by Asterisk when Richard finishes his call.
Scenario 6: Mark calls Richard, and Richard does not respond within the 20 second dial timeout. Mark requests call completion. Richard does not use his phone again for another 4 hours (144000 seconds). Since Mark has the default ccnr_available_timer of 7200 seconds set, Mark will not be automatically recalled by Asterisk when Richard finishes his call.