Server just stopped working - can't get calls (fails to register with phonebooth.net)

The system fails to register, it says its in status “Request Sent” but it never seems to get a response.

Firewall test passes, iptables is off and has been flushed, there is no trouble connecting to the box via other services, pinging the trunk1.phonebooth.net server gets a reply, so I don’t know why it won’t register?

Without a lot more information, and if all was working well then suddenly stopped, I would be contacting my provider which appears to be phonebooth.net. from the PBX CLI, can you ping your provider? Does it resolve?