I have a linksys router that does NAT. In order for me to make VoIP calls from Internal Gateway I setup Port forwarding.
VegaStream Gateway:
application: Port #:
SIP 5060-5070
RTP 2000-3000
2 way audio isn’t an issue when setup properly. Please be aware not only is the setup for SIP similar, but you also have to take into account the Port numbers vary by Manufacture.
I hope this helps.
Most gateways & phones have a config option for such applications.