What is SIP ALG, and why should be disabled on most routers ?


SIP ALG stands for Application Layer Gateway and is common in many commercial routers.

It inspects VoIP traffic to prevent problems caused by firewalls and, if necessary, modifies the VoIP packets.

Many routers have SIP ALG turned on by default and, whilst its purpose is to assist users who have phones running over the internet, in many cases it actually causes more problems than it solves!

How does SIP ALG affect VoIP?

As SIP ALG can modify the destination of the VoIP packets, it causes an interruption between the phone and the service provider.  This can result in unexpected behaviour, such as:

1. Phones not registering
2. Incoming calls failing
3. One-way audio
4. Phones not ringing
5. Calls going straight to voicemail

How do I turn off SIP ALG?

Almost all routers have a web interface. You will need to log into this interface to access the settings and disable SIP ALG.

An example is shown below.

Your router manufacturer’s user manual will often run through how to disable SIP ALG, to allow you to make this change.

If you need any further information, or support please get in touch with the Team today