What is SIP ALG?

SIP ALG (Application Layer Gateway) is a feature included in some routers and firewalls. It’s intended to help with NAT traversal (allowing devices behind a router to communicate with the outside internet).

Why SIP ALG is problematic for VoIP

Although well-intentioned, SIP ALG implementations on many routers are flawed. They attempt to modify SIP (Session Initiation Protocol) packets, which are essential for setting up and managing VoIP calls.  This modification by SIP ALG often breaks core VoIP functionality, leading to issues such as:

  • One-way audio (you can hear but not be heard, or vice-versa)
  • Dropped calls
  • Registration problems (phones unable to connect to the VoIP service)
  • Inability to call certain numbers

The Solution

The best solution is to disable SIP ALG entirely within your router’s settings. This prevents it from interfering with your VoIP traffic.  Not all routers with SIP ALG will cause issues, but it’s a common culprit when VoIP problems arise.

When SIP ALG Might Be Needed

In rare scenarios with specific network setups, SIP ALG could be necessary. These are usually situations where a VoIP client is behind a ‘symmetric NAT’ and your VoIP provider doesn’t offer a way to resolve this on their side.

Bottom Line

If you’re experiencing VoIP problems, disabling SIP ALG is the recommended first troubleshooting step. If that doesn’t solve the issue, you’ll need to investigate other potential causes.  For more details about SIP ALG, check out Voip-Info.org

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