Interesting set of findings from an Interop Labs' test of VoIP gear run on the Interop Las Vegas show's earlier this month.
Network Address Translation, or NAT, for short, (as shown on th Cisco site, above) can break VoIP. NAT does this by its nature as a procedure that masks private IP addresses from public view. Yet by doing this, NAT makes it impossible to set up Internet-delivered, SIP-based calls to devices with private IP addresses.
Interop Labs said the best solution is to get rid of NAT. If that isn't feasible, there are two basic alternatives:
Install a server outside the NAT device. This would be a server that can keep track of where packets are initiated from and then move them through the NAT.
The other practical approach would be to install a SIP proxy server capable of "ignoring" the public addresses of VoIP packets while, at the same time, detecting the actual addresses within those packets.