Thought i'd give some extra info.

There is an issue on some TW server's with it's WAN side ip adress. What happens is that the TW server send it's private WAN NIC IP adress in the TCP/UDP headers instead of it's public IP. That way the TW sets can't reach the TW server. This causes the sets to boot and start up normally, but no audio when you set up a call.

If this happens you have to 'hack' into your TW Server and force it to send it's public IP.

I have a document describing the process if you need it.