It seems Application Layer Gateways (ALG) can cause one-way audio problems for NuFone’s SIP-based VoIP services. For reasons that we have not been able to determine, if your router has an ALG that supports SIP ensure that you DISABLE it. We take the necessary steps to correct any NAT based headers on our SIP Proxy. In fact, your VoIP device shouldn’t try to fix the NAT situation whatsoever since our SIP Proxy is quite good at detecting and correcting NAT situations.

For example, tonight while we were testing our new West Coast POP, we discovered that no matter what we did with our SIP Proxy we would always get one-way audio when the ALG was enabled. The moment we disabled the ALG and let our system munge the NAT headers, audio would flow in both directions as expected. The router that we were testing with is the DLink 4300 with firmware version 1.6.

Hope this helps those that have reported one-way audio problems when using SIP with NuFone. If anyone has a clue about ALGs and what they expect from SIP Proxy’s please feel free to give us a clue.