What is SIP ALG and Why does it Affect my Phone Calls?
This page is part of the Network and QoS guides.
SIP ALG
What is SIP ALG and Why does it Affect my Phone Calls?
In the world of Voice over IP, the term SIP ALG is thrown around a lot, but have you ever wondered what it is? In this article, you will learn what SIP ALG is and how it affects our system.
Technical Definition: SIP ALG stands for Application Layer Gateway and is common in all many commercial routers. Its purpose is to prevent some of the problems caused by router firewalls by inspecting VoIP traffic (packets) and if necessary modifying it.
What does SIP ALG do on a Phone Call?
SIP ALG inspects VoIP packets, meaning it is constantly modifying and editing the packets over the internet that carry your voice. When a phone call connects over the internet, it opens up ports between you and the caller, almost like a tunnel system. Your voice packets on that call can only flow through the connected tunnel system. SIP ALG modifies these tunnels while on a phone call, which causes your voice packets to get lost, take a detour, or come through in complete.
When your phone call is completed, those tunnels (ports) get closed off, allowing the phone call to end. When SIP ALG is enabled, those tunnels will continue to remain open, or only some tunnels will close, while others stay open. In other cases, old voice tunnels will stay open indefinitely, causing voice packets to flow to un-desired devices or locations.
What are the Symptoms caused by SIP ALG?
Ghost Calls: Ghost calls are caused by left open voice tunnels (ports). As described above, when a call ends, all tunnels are supposed to be closed. Well, SIP ALG is on, sometimes it closes one callers tunnels, but not the other, this is what causes whats known as a ghost call. The system is recognizing a tunnel is open, so its ringing a phone, but there really is no active call, which is why when you pick up the receiver it will be dead air.
Calls Bouncing Between Extensions: SIP ALG changes the voice ports as it sees fit, which is why sometimes a call will ring for half a second at one extension, and then bounce to another for half a second, and so on.
Answered the call, but it kept ringing on other extensions: This happens because SIP ALG will not close the excess voice tunnels (Ports) that were opened on the initial broadcast of the call. Meaning even though the call is connected on one device, SIP ALG keeps trying other devices.
How do I turn off SIP ALG?
SIP ALG is a firewall setting usually stored on the internet provider's modem. To disable it, you need to call your internet provider and request it be disabled. If you are still having these issues and your internet provider confirms SIP ALG is disabled, then the firewall in your router may have it enabled. You will need to research where to find this setting in your router and disable it to resolve your issues.