The CallManager can send a SCCP message to the phone to terminate a call.
The 7920 will not drop a call unless pressing endcall button.
Can even roam out of AP coverage for multiple minutes and can return where the call can be re-estabilished (assuming other side didn't hang up already).
So is this incoming call to the 7920?
Does it ring? If so, then once you select answer, the phone will sent what IP and UDP port to send RTP to. Once rx info about the other device, then may have to ARP if the address is not already known.