What is SIP ALG?
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 (packets) and if necessary modifying it.
Many routers have SIP ALG turned on by default.
How can it affect VoIP?
Even though SIP ALG is intended to assist users who have phones on private IP addresses, in many cases it is implemented poorly and actually causes more problems than it solves. SIP ALG modifies SIP packets in unexpected ways, corrupting them and making them unreadable. This can give you unexpected behaviour, such as phones not registering and incoming calls failing.
Therefore if you are experiencing problems we recommend that you check your router settings and turn SIP ALG off if it is enabled.
How do I turn off SIP ALG?
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").
Please note that if you have a TG582n or TG589vn for our DSL service then SIP ALG will have no impact and the device is plug-n-play.
I have disabled SIP ALG but I'm still experiencing problems...
If you are still having problems after disabling SIP ALG, please check your firewall configuration.