Yes, needing port forwarding is likely a router/firewall issue, or the registration is not happening often enough to keep the SPI window open so VOIPo can ring the phone. (Failure to ring to one of 5060-5080)

This thread link endorsed port forwarding with even Nitzan buying into it.
http://www.dslreports.com/forum/r220...for-prime-time

They talked 35000-65000 forwarding. That would be the voice streams, and that seems a desperate measure. That ... firewall issues? A more rare need, and I will not speculate cause/effect.

A simple network, with decent router - not needed.