SIP Trunks and Router

I am running an 8-line Voicent Broadcast By Phone enterprise system. My SIP provider is Bandwidth.com, which uses IP-authentication. I have 30 SIP trunks which are used on a line-rotation basis. I have cable broadband with a static IP assigned to my location. The system works perfectly when the computer running it is directly connected to the cable modem. However, if I introduce a router, things become difficult. The router is a Netgear WNR3500. I have disabled the SPI firewall, established a default DMZ for the computer that is running voicent gateway, and established port forwarding on port 5060 as well as 9155-9300. I was getting a 408:request timeout error, so I disabled the SIP alg built into the router. Now the system rings out but will not play back any audio once the phone is answered. This leads me to believe that the voice signal is not getting back to the computer through the router. Do I need to open additional incoming ports via port forwarding or triggering?

Looks like you are missing the audio ports forwarding. The 5060+lineNumber TCP ports are the SIP message port, and 9156+lineNumber UDP ports are the initial RTP ports for SIP audio streams. These UDP ports are negotiated by the two SIP endpoints and are dynamically set. So you may need to open additional UDP ports for the SIP audio.

One way to find these ports is to check the SIP message. You can use a free program called wireshark to watch your SIP traffic. The “SIP OK” message in response to “SIP INVITE” should contain information about the audio ports.