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Disabling SIP ALG

Disabling SIP-ALG is an essential part of setting QoS on a router and optimizing it for your VentureTel service. Many ALGs (including Cisco's) have bugs which cause call flow and registration failures. Some ALGs (including Cisco's) intermittently miss some packets, or in the case of fragmented packets, do not even examine and change headers.

When SIP-ALG is enabled, session boarder controllers (SBC) determine the endpoints are publicly addressed and therefore do not need frequent registration refreshes to keep the firewall port open between SBC and the endpoint. In this case, the firewall can close the port between the VoIP Zone SBC and endpoint, causing an inability to receive incoming calls. The most common issues that result from enabled SIP-ALG with VentureTel include:

  • Outbound call status stuck in Dialing...
  • An inability to field incoming calls (call continues to ring and cannot be answered).
  • IP Phones lose registration
  • Presence status and other BLF keys not displaying correctly.

Additional SIP-ALG information and settings can be found at

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  1. Joel Eskelsen

  2. Posted
  3. Updated