what could cause a cancel?
 

Here's the scenario. Caller's (A) ATA is not behind router. Callee's (B) ATA behind a router. B has set up all portforwarding rules for both SIP and RTP ports leading to his ATA behind router. Also on B's ATA "HANDLE VIA addr" is turned on to provide external router IP for all subsequent messaging.

When A is calling B, an invite is sent out from A to B. I proceed to see ringing and 200 OK response from B. Just at this point, a SDP media message sent from B in this 200OK message states clearly that IP address is that of the router and RTP port is within range of my forwarded ports on the router. But at this moment, the SIP proxy immediately shouts back a CANCEL message to B as if A had cancelled the call when B picks up the ringing phone. So the call never got an ACK from A and connection is never made. Both phones are using G.711u. I wonder what could cause the cancelling. Is there something the proxy detects that invalidate the call?

I check my log carefully and so far I do find something suspicious. There are in fact two invites being sent out for B, each with a different branch number and almost identical

(1) userB@internal_IP:5060
(2) userB@external_router_IP:5050

When B answers the phone, somehow a CANCEL message is dispatched to cancel the invite sequence for (1). That in itself seems to kill the connection as well. There's no CANCEL message sent for (2) but there's no ACK sent for (2) either so I suppose the act of CANCEL for (1) has killed off (2) for a certain reason.

So far I still have not known exactly why that device behind the router registers twice, once with an internal LAN address and another with an external router IP. But that's not the issue here. Should the CANCEL message for (1) kill off that of (2) as well?