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What Is SIP ALG?

Why SIP ALG is the bane of our lives

Have you encountered the following with your VoIP calls?

  • One way or no way audio
  • Calls not connecting
  • Silence part way through a call
  • Handsets or softphones continuing to ring even after answering
  • Unable to call or transfer internally
  • Softphones unable to register

These are some of the most common symptoms caused by SIP ALG and for that reason should you raise a VoIP issue with our support team we may ask you whether SIP ALG is enabled on your router or we may ask for test calls so we can check for ourselves if you are unsure.

Our recommendation is always to have SIP ALG switched off.

So what is SIP ALG?

SIP ALG stands for Application Layer Gateway and is common across many routers and firewalls. Its aim is to prevent some of the problems caused by router firewalls by monitoring VoIP calls (packets of data) and if necessary, modifying it.

During Covid-19 and the sudden need to work from home we have found most home broadband providers, SKY, TalkTalk, EE have SIP ALG enabled by default.

How can it affect VoIP?

Even though SIP ALG is intended to assist VoIP users in many cases it is implemented poorly and causes more problems than it solves. VoIP providers like KAT Communications often find that the SIP ALG modifies SIP packets in unexpected ways, corrupting them and making them unreadable.

This can give you unexpected behaviour as per above, such as phones not registering and incoming calls failing and frustratingly it does not happen all the time!

How do I turn off SIP ALG?

All good business broadband routers or firewalls will allow this ‘feature’ to be deactivated. Disabling the feature will depend on the make of model of your router. If your router is provided by KAT Communications, then SIP ALG is turned off by default.

Our advice would be to speak to your router and broadband provider. Typically, home broadband providers will resist changing the settings or will be required to access the router remotely to amend.

Even worse some broadband providers ‘freebie’ routers are not able to have the setting disabled.

I have disabled it, but I am still experiencing problems?

If you are still having problems after disabling then contact the KAT Support team via 01302 260195 or who will be able to support you.

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