I'm using 3CX SIP softphone v4.0.13679.0. My phone is registering with a registrar/proxy by way of a Cisco VPN connection. I noticed in the REGISTER request sent by the phone that the IP address of the Via header is set to my computer's IP address associated with the Cisco VPN connection, and the IP address of the Contact header is set the IP address of the Cisco VPN device. What would cause this to happen? I suspect this difference in IP addresses is causing call connectivity issues. When I replace the 3CX softphone with an XLite softphone I have no call connectivity issues. When I look at the Via and Contact headers of the REGISTER request sent by the XLite softphone I noticed both headers have the same IP address (i.e. my computer's IP address associated with the Cisco VPN connection). Is there a way to configure the 3CX softphone to use the same IP address in both the Via and Contact header of its REGISTER request?