Thanks Jan, I should have made myself a little bit clearer; I am using the RTC 1.2 API (http://msdn.microsoft.com/library/default.asp?url=/library/en-us/rtcclnt/rt c/real_time_communications_rtc_client_start_page.asp) What I find the most interesting is - http://msdn.microsoft.com/library/default.asp?url=/library/en-us/rtcclnt/rtc /traversal_of_upnp_enabled_nats.asp specifically the last sentence of the third paragraph. Why I can't provide it my external IP address and port on the NAT is annoying! I wonder if it would be possible to modify the REGISTER message on the way to the SIP server and change the IP:Port to my external NAT address mapping - or get NATHelper to do this for me.