When discussing VoIP issues with our support team we may ask you whether SIP ALG is enabled on your router. This article explains what it is, how it can affect VoIP and why we recommend that you turn it off.
SIP ALG stands for Application Layer Gateway and is common in many commercial routers. It intends to prevent some of the problems caused by router firewalls by inspecting VoIP traffic and if neccessary, modifying it. Many routers have SIP ALG enabled by default.
While SIP ALG is intended to assist users who have phones on private IP addresses, in many cases it is implemented poorly and causes more problems than it solves. We often find that SIP ALG modifies VoIP traffic in unexpected ways, corrupting it and making it unreadable. This can give you unexpected behaviour such as phones not registering, calls failing or one way audio.
The method to disable SIP ALG will depend on your router. In the first instance we recommend checking the manufacturers instructions that came with your device. Alternatively you can check support forums for advice.
If you are still having issues after disabling SIP ALG please contact our support team who will be happy to help you.