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  • Why does SIP ALG stop my router making VoIP Calls?



SIP ALG IS A "FEATURE" RUNNING ON A FIREWALL AND/OR ROUTER. its is an Application Layer Gateway that's purpose is to provide protection to private IP addresses on a Network Address Translated (NAT'D) network.

Most good firewalls allow this "feature" to be disabled off in their settings and a good premium business router like the Draytek series will have it disabled by default.

A poorly implemented SIP ALG it can be a problem rather than a solution. We at Jibba Jabba and other VoIP providers find that the SIP ALG modifies SIP packets in unpredictable ways, corrupting them and rendering them unusable.

The signs of a SIP ALG problem are:

  • Non registering Hardware Phones and Softphones leading to incompatibility with our platform.
  • Hardware Phones and Softphones appearing to be registered but unable to receive call control data in or out to make your phone ring or transfer between callers.
  • One way audio, created by the SIP ALG which stops our data reaching your network.

A SIP ALG problem isn't always obvious. Firewalls invoke algorithms that will often be deciding when the SIP ALG should inspect traffic, perhaps on your first VoIP calls, that could follow a specific set pattern of traffic.

We find see poor implementations are most common on 'free' routers that come with broadband connections. We would always recommend replacing them with a pupose built business grade router like the Draytek series to eradicate the most common problems.

On these 'free' routers, we quite often see SIP ALG continue to interrupt traffic even though it has been set to "off" on the router with the router being unable to implement the settings it has been given.


To diagnose wether your router/firewall is affected by SIP ALG, Type into Google Search the make and model of the offending router/firewall and the words 'SIP ALG'. VoIP uses standard protocols, so you can be assured if it's a problem for you and us, it will be be documented. There will plenty of commentary on if it can be disabled and how to do it. If you do disable SIP ALG, be sure to reboot the router following its removal immediately before rebooting each hardware phone / softphone on the network too. This is vital to ensure both SIP ALG has been disabled and to ensure your devices have an updated connection to our platform; we can then reach you on your network.


It is possible to counteract SIP ALG on some routers by using encryption on both their SIP (Activating TLS) and RTP traffic (activating SRTP) on their devices. However not all hardware phones and softphones permit TLS or SRTP, try looking for these settings on your phones and activating them on both your phone and in your dashboard. Be wary though it's all too easy to create problems than you solve doing this, make sure you read up and understand encryption VERY carefully first.


Article by Ashley Harris on: 24/4/20




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