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SIP ALG stands for SIP Application Layer Gateway.  You will find it on many commercial and residential Firewalls, Routers, or Modems.  It is a NAT tool that inspects SIP Messages and transforms Private IP addresses and Ports to Public IP Addresses and Ports.  Modern VoIP does not require SIP ALG and may cause issues with your phones in many situations.

If SIP ALG is enabled, we recommend turning it off on your router.  To test your router to see if SIP ALG is enabled, run this SIP ALG Test.

The result should show SIP ALG IS NOT DETECTED.  If it is detected, you or your IT vendor must log into your router and disable it.