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What Is SIP ALG and Why Is It Affecting My VoIP Calls?

 What Is SIP ALG?

SIP ALG (Session Initiation Protocol Application Layer Gateway) is a feature built into many internet routers and firewalls. It is designed to modify SIP (VoIP) traffic as it passes through the network.

Although it was intended to improve VoIP compatibility, SIP ALG often has the opposite effect and can interfere with VoIP services.

Why Does SIP ALG Cause Problems?

SIP ALG can rewrite or block parts of the SIP signalling used by your phone system, resulting in communication issues between your equipment and our network.

This can lead to:

  • One-way audio (you can hear the other party, but they cannot hear you, or vice versa).
  • Calls that fail to connect.
  • Calls that disconnect unexpectedly.
  • Phones failing to register.
  • Delayed call setup.
  • Missing inbound calls.
  • Poor call reliability.
How Do I Know If SIP ALG Is Enabled?

Many routers have SIP ALG enabled by default.

Some internet service providers also enable SIP ALG on routers they supply, even if there is no visible option to disable it.

If you are unsure, check your router's administration settings or contact your IT provider or internet service provider.

How Do I Disable SIP ALG?

The process varies depending on your router manufacturer.

Generally:

  1. Log in to your router's administration page.
  2. Locate the SIP ALG or Application Layer Gateway setting (often found under Firewall, Security, NAT, or Advanced settings).
  3. Disable SIP ALG.
  4. Save the changes.
  5. Restart your router and your VoIP devices.

Some routers may require a firmware update or command-line configuration to disable SIP ALG.

We Recommend Disabling SIP ALG

EveryDay VoIP recommends that SIP ALG is disabled on all routers used with our VoIP services unless specifically advised otherwise by our Technical Support Team.

Disabling SIP ALG resolves many common VoIP issues and allows SIP traffic to pass through your network without unnecessary modification.

Still Having Problems?

If disabling SIP ALG does not resolve the issue, please raise a support ticket through your Customer Portal and include:

  • Your Subscriber ID.
  • Your registered email address.
  • Your router make and model.
  • Your internet service provider.
  • A description of the issue.
  • The date and time the problem occurred.

Providing this information helps our engineers diagnose your issue more quickly.