We have NAT ed Cisco 7960 phones that have been used with other IPPBX systems and VOIP providers directly with 100% success. We have battled with registering to 3CX on a Public IP. THe phone is behind a NAT and not the PBX. IN the case if the CIsco Phone , and this did not happen with Snoms, an ethereal capture from the 3cx machine revealed it was sending all RTP packets to the LAN IP of the phone, and NOT the public IP . Those packets originating from a Public IP are clearly destined for 192.168.1.X , a non routable LAN IP. OF course on the Snom we had to enable STUN , which is not an option for the Cisco. I think STUN resolves the public IP I might point out that the phone registers, however I suspect that the source for the RTP IP is derived differently than the IP for SIP packets. I originally did the Capture from the 3cx machine to look to see the destination ports, and much to my surprise I found them destined for the LAN IP, not the publicly routable IP Any ideas?