I have a new Asterisk 11.4 installation on a Ubuntu 12 64-bit server which I am playing about with at the moment. The server is located in the cloud with a public IP address so not NATing, just firewall.
I have 2 remote phones behind a router, one is a Bria app on my iPhone which runs fine and the second one is a Cisco 8945 IP Phone. The IP phone connects to the server but I am unable to call it from the other extensions. I can call from it but I get no audio and receive this message on CLI:
[Jul 10 14:48:03] WARNING[C-00000008]: app_dial.c:2437 dial_exec_full: Unable to create channel of type 'SIP' (cause 20 - Subscriber absent)
[Jul 10 14:54:00] ERROR: tcptls.c:451 ast_tcptls_client_start: Unable to connect SIP socket to 10.10.11.13:56478: Connection timed out
Both of these have nat=comedia set under the extension config bit of sip.conf file.