What I mean by that, is say you have a direct data circuit to a SIP provider, with no internet access on the circuit, like a closed VPN. Then connect to the SIP proxy internal address across the data circuit, to provide outside connectivity to your ip-pbx?
Or is this a fundemental voilation of SIP's protocol, being peer to peer?
This eliminates the possibility to use stun, and I am not sure that having the ippbx not accessible at all from the internet would cause problems or not.
It would certainly help with hacking though, given that the 5060 ports are not accessible...