[Serusers] Cisco pstn gw ignoring BYE from ser

In the following scenario, it seems that ser may not be sending the BYE to the right port on the cisco, is that possible? The cisco is not registered with ser, it is a trusted IP. The DID is an alias for my softphone UID. This only happens for pstn-voip calls, when calling voip-pstn, ser always talks to the cisco on port 5060 and the BYE is obeyed, whichever end sends it first. call scenario dial DID from pstn phone cisco:51339 -> ser:5060 INVITE ser:5060 -> cisco:51339 100 trying ser:5060 -> cisco:51339 180 ringing softphone ringing ser:5060 -> cisco:51339 200 OK softphone answered cisco:53924 -> ser:5060 ACK call in progress, 2 way audio I hang up the softphone ser:5060 -> cisco:51339 BYE softphone says "hanging up" ser:5060 -> cisco:51339 BYE ser:5060 -> cisco:51339 BYE ser:5060 -> cisco:51339 BYE ser:5060 -> cisco:51339 BYE ser:5060 -> cisco:51339 BYE ser:5060 -> cisco:51339 BYE ser:5060 -> softphone:5060 TIMEOUT softphone says "hung up" pstn phone still off hook, call up still i hang up the pstn phone cisco:50580 -> ser:5060 BYE ser:5060 -> cisco:5060 OK ser:5060 -> cisco:51339 BYE So the cisco has used 3 different ports during this call, one for the INVITE, which ser then uses to send replies back to, but the ACK comes from a new port, and then the eventual BYE comes from a 3rd port. I can understand how the cisco tries not to be stateful and uses different ports for each message, but how is ser supposed to communicate back to it if not on the port used by the original INVITE? Perhaps it should only talk to the cisco on port 5060? If so how do I make it do that? Is the cisco misbehaving by using many different ports when it originates the sip call? Is that a known IOS bug perhaps? Help and wisdom appreciated, Jon