SIP ALG stands for SIP Layer Gateway. It is also called SIP helper, SIP service, SIP stack. SIP ALG is a piece of software in a router that modifies IP addresses to help incoming VoIP calls.
Although SIP ALG is intended to help users with their own IP address, unfortunately SIP ALG often breaks more than it solves. SIP ALG modifies SIP packets in unexpected ways, damaging them and making them unreadable. This can lead to unexpected behaviour, such as VoIP phones not registering and incoming calls being dropped.
It is therefore very important that SIP ALG is turned off in the router. If this is not done, problems may arise in which the account cannot register, or calls cannot be made (or sometimes things like one-sided audio etc.).
The following problems can occur when SIP ALG is turned on.
- One-sided audio or no audio
- Calls being disconnected
- Problems with call forwarding
- Putting calls on hold or removing them from hold does not work properly
- Not all stations in a ring group are ringing
- Phones continue to ring after being answered