SIP ALG stands for Application Layer Gateway and is a common component in many commercial routers. It is intended to prevent by inspecting VoIP traffic (packets) and if necessary, modifying it.

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Many routers have SIP ALG turned on by default which can negatively affect VoIP.

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Even though SIP ALG is intended to assist users with phones on in many cases it is implemented poorly and actually causes more problems than it solves. Many providers often find that the SIP ALG modifies SIP packets in unexpected ways, corrupting them and making them unreadable. This can cause unexpected behaviors, such as phones not registering and incoming calls failing.

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If you are experiencing problems, we recommend that you check your router settings and turn SIP ALG off if it is enabled.

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The method to disable SIP ALG will depend on your router. In the first instance, we recommend checking the manufacturer’s handbook that came with your device. Alternatively check the internet and support forums for advice (type your router make and model into a search engine, along with “disable SIP ALG”).