Now we are going to work with call forwarding. There are usually three kinds of call forwarding. This forwarding is important for voicemail operations.
user_preferencestable. The SIP router will fork the call, creating a new leg to the new destination. The phone with call forward configured won't even ring, whether it's registered or not.
failure_routefeature to intercept the "486 Busy" message and create a new leg by sending the INVITE request to the final destination.