First a little background on SIP ALG (Application Layer Gateway). It is a security component of a router or NAT that allows VoIP traffic to pass through from the private to the public and vise a versa through the firewall when NAT and NAPT is being used. The SIP ALG inspects the traffic and then repackages it as it allows it to continue on to its destination. Many routers incorporate this protocol, however unfortunately after the inspection some of the information is lost or rearranged which results in the return RTP not making it to the final destination inside the LAN. The result is one-way audio where the caller can hear the person inside the network behind the router, but that user cannot hear the person on the outside. The RTP data never makes it back to him.
SIP ALG on a router does the following:
This setting is found primarily on a SonicWall and are basically SIP ALG and is well known to cause issues with some VoIP services. These issues can be one-way audio, phone registration issues, or even dropped calls. Most VoIP carriers will request the user to disable (or uncheck Enable) for SIP Transformations and enable consistent NAT (Network Address Translation) on a SonicWall if these issues are occurring.
Most routers have SIP ALG enabled by default. Disabling this setting is easy and should be attempted as a way to troubleshoot and resolve some VoIP issues including one-way audio. Below are two screenshots showing where on a Linksys WRT1900AC and on a SonicWall they can be found. There are also screenshots of where to disable SIP ALG on a Netgear AC1900 and a version of Tomato firmware.
Most good routers have advanced routing rules that can help accomplish many obstacles, such as Port Forwarding and other advanced routing features.
As shown, the Sonicwall includes a setting to Enable consistent NAT (Network Address Translation). This setting can help ensure that the NAT address remains the same, helping voice traffic (RTP) to find the correct endpoint.
The Netgear Nighthawk series of routers are fast and can handle numerous devices and are good for networks that use VoIP. The AC1900 offers excellent value and performance.
Depending on the version of Tomato, SIP ALG cab be found under Advanced then Conntrack/Netfilter in the Tracking/NAT Helpers section. If you find SIP checked then SIP ALG is enabled. Uncheck it to disable it.