What is SIP ALG?


SIP ALG stands for Application Layer Gateway and is common in many commercial routers. 


It intends to prevent some of the problems caused by router firewalls by inspecting VoIP traffic (packets) and if necessary modifying them.


Many routers have SIP ALG turned on by default.


How can SIP ALG affect my VoIP service?


Even though SIP ALG is intended to assist users who have phones on private IP addresses, in many cases it is implemented poorly

and actually causes more problems than it solves as the SIP ALG modifies SIP packets in unexpected ways, corrupting them and 

making them unreadable. 


This can cause issues with your VoIP line such as:

  • Calls dropping after a set period of time
  • Calls dropping when trying to retrieve the call from hold
  • Calls dropping when being transferred
  • Calls not being received
  • Unable to make outbound calls
  • Calls with no audio
  • Calls with one way audio
  • Other extensions continue ringing after call has been answered
  • Call connectivity issues - a call is transferred but the person cannot hear the caller at the other end.


Therefore if you are experiencing problems we recommend that you check your router settings and turn SIP ALG off if it is enabled.


How do I turn off the SIP ALG setting on my router?


Not all routers offer the option to disable SIP ALG.  In the first instance we recommend checking the manufacturers handbook.  


Some routers do not allow you to disable the SIP ALG such as:


Virgin Superhubs, BT HomeHubs (prior to version 6) and Friz Box


TelNet commands must be used to disable SIP ALG on Technicolor, Thomson, SpeedTouch, some Draytek and some ZyXEL 

routers.


Please click here for some instructions for turning off the SIP ALG setting on some common routers. 


If you are unable to find information for your router then we recommend you check the internet and support forums for advice by typing

your router make and model into a search engine with the text 'disable SIP ALG'.


Unable to Disable SIP ALG - Try Changing Ports


If you are unable to disable the SIP ALG setting on your router then you could try changing the port to or from either 5060 or 5065 or 

changing to 4072. 


Please click here to see how to change the port setting.


Use a STUN Server 


You could try using a stun server to get around the SIP ALG if you are unable to disable this.   Please click here to find out more. 



Does it affect anything else if I disable the SIP ALG settings on my router?


No not at all, your internet web access is routed differently to VoIP packets for calls. Therefore if you disable the SIP ALG on your router the internet will not be affected.


Please note - if you are disabling the SIP ALG on your router please make sure you 'save' the settings and then 're-boot' your router for the change to take effect.