📞 Disable SIP ALG on Sophos Firewall

Step-by-step guide to disabling the SIP ALG module on a Sophos Firewall to fix VoIP telephony issues (dropped calls, no audio, instability).

Why disable SIP ALG?

In some cases, VoIP telephony may experience issues such as no audio, dropped calls, or instability. A common cause is the activation of SIP ALG (Application Layer Gateway) on the firewall.

SIP ALG automatically modifies VoIP (SIP) packets, which can:

  • alter IP addresses and ports
  • disrupt signalling
  • cause audio issues (RTP)

In modern VoIP environments, this feature is generally unnecessary and can cause malfunctions.

Disabling procedure

1. Access the CLI console

Two options:

  • via the Web interface (CLI Console)
  • via SSH on the firewall

2. Unload the SIP module

Run the following command:

system system_modules sip unload

3. Check the status

Run:

system system_modules sip show

The SIP module should no longer be active.

Good to know

  • On Sophos, SIP ALG is not always visible in the graphical interface
  • It operates at the system level via a dedicated module
  • Disabling via CLI is the recommended method

After disabling

It is advisable to:

  • test inbound and outbound calls
  • verify audio in both directions
  • check the stability of SIP registrations

When troubleshooting telephony issues, checking SIP ALG should always be the first step.

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