>but their service doesn't work with IPTables NATs, so I had to move over to FWD

Hmm, I thought of direct IP-Dialling, without any Provider between the phones, maybe in a local net without Internetconnection. And I have a router for which I can forward some ports to an internal IP (should be no problem for SIP). But, unluckily, I hadn't the chance for testing...
As I'm aware, SIP should use only few and fixed ports (just the opposite to H.323,which is very unhandy in this case). So one device should call another device by hitting it's IP and connecting it directly at the SIP-Port (something in the 5000-Range, I think).
Are connections to Netmeeting possible? The H.323-capable phones should be good for this.
Is there someone who is willing to call me via Internet with such a phone at a fixed time for a short test?

Rolf