It appears that the phone system server is sending it's private IP (1192.168.1.103) in the sip heaaders. The remote IP server can't therefore get back to it. I am assuming that the 3cx server uses STUN to determine what it's public IP should be. How do I make the server use STUN to get it's ppublic IP and how do I test to see if stun is working from the 3cx server. I've done the firewall test and it worked fine. I've also set port forwarding so that the external sip server can get through to my 3cx erver (ncoming calls work fine, but ourgoing faail to establich a SIP connection) Thanks for any advice o help.