The briefly explains what SIP ALG is and why it should be disabled.
Refer to the required firewall changes for additional details.
What is SIP ALG?
SIP ALG stands for Application Layer Gateway, and is common in many commercial modems and 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. Celito and other providers often find that the SIP ALG modifies SIP packets in unexpected ways, corrupting them and making them unreadable. This can give you unexpected behavior, such as phones not registering and incoming calls failing.
Therefore, if you are experiencing problems, we recommend that you check your router or modem settings and turn SIP ALG off if it is enabled.