Samsung Ubigate Gateway - minor issue with Sip-X

Hi all, I've recently installed a new Samsung Ubigate gateway router to allow me to connect Sip-X 3.10 into my existing TDM switching systems. The connections have been made and calls are flowing back and forth, but on Sip-X almost all the calls through the gateway are showing as being active forever. It looked like the gateway was not sending the BYE message that trips Sip-X to consider the call ended.

When I ran a debug on the Samsung however, it was clearly sending out the BYE message, so I stuck on Wireshark and did a capture of a few calls to see what was going on. IP to IP calls are flowing beautifully, with a text book call flow (100, 180, 200, BYE). However, when I call through the gateway, I get additional messages which cause problems.

After the initial invite it now comes back and asks for proxy authentication, which is acknowledged and another invite is sent out. At the end of the call when the BYE message is sent out from the gateway, the phone returns a 481 Call/Transaction does not exist error code, and the call never completes successfully in Sip-X.

Anyone had this type of error before? I think the re-authentication is causing a new call number to be raised with the server, and so they become out of synch with each other and the BYE message is discarded as being for the wrong call. Does that sound logical? Help?