Unexpected call failures and registration problems


If you are experiencing problems where phones are not registering or incoming calls are failing, then the issue may be that SIP ALG is enabled in your router. The following are a list of issues that you may experience due to SIP ALG:

  • One-way audio on an incoming call
  • Dead air/dropped call on an incoming call
  • Failure to transfer an incoming call
  • Unable to put a call on hold or park
  • Unable to resume a call or unpark
  • Inbound faxes failing
  • One phone in a group not ringing
  • Phones are not registering

What is SIP ALG?

SIP ALG stands for Session Initiation Protocol Application Layer Gateway. It is intended to prevent some problems caused by firewalls by inspecting VoIP traffic and if necessary modifies the VoIP packets. Many commercial routers have SIP ALG turned on by default.

What is the impact of SIP ALG?

SIP ALG modifies SIP packets, which controls voice calls and faxes.  In most cases, SIP ALG is poorly implemented and modifies the SIP packets in ways that either corrupt the packets or removes pertinent information from the packets. Therefore problems caused by SIP ALG can be varied and unpredictable.

How to turn off SIP ALG?

Instructions to turn off SIP ALG is specific to each router. You will need to refer to the manufacturer's handbook for your device.

Note that some manufacturers refer to their SIP ALG implementation by various names. For example, Sonic Firewall products sometimes refer to it asSIP Transformations. Cisco routers typical require disabling of both their SIP ALG as well as their Stateful Packet Inspection (SPI) functionality.
Note that similar issues can be caused by other network configurations such as intellignet Layer 2 switches that detect SIP signaling packets and automatically modify those packets to either tag them with VLAN information, mark them with DiffServ prioritization packets, or handle them in some other, unexpected way. For example, Netgear Prosafe PoE switches may require disabling of their Auto-VoIP feature.
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