SIP (Session Initiation Protocol) ALG (Application Layer Gateway) is an application within many routers. It inspects any VoIP traffic to prevent problems caused by firewalls and if necessary modifies the VoIP packets. Routers will often have SIP ALG activated by default.

How can it affect SIP calls and registration?

If SIP ALG is activated, it can stop your SIP trunks from:

  • registering to the platform
  • making and receiving incoming or outgoing call
  • One way audio transmission

SIP ALG must be disabled

If your router and/or firewall are "SIP Aware" / has a SIP ALG enabled, then this functionality must be turned OFF so that the device does not interfere with any signalling.

BT Disclaimer

Your BT SIP Trunks service together with a correctly dimensioned BTnet access service has been designed to appropriately support a quality voice service alongside the use of data. In order to maintain the quality it is essential that the internal network components suitably support the service for delivery of your voice calls. If this is not implemented correctly then this may result in a degraded service. If faults are reported to BT which are found to be caused by customer equipment (i.e. equipment which that is not part of BT's network and which is owned or controlled by the customer) then BT may apply a charge.

The information in this document is provided for general guidance only. It is recommended that your firewall maintainer, switch maintainer or IT consultant is consulted on all matters relating to your communications network including, but not limited to, PBX configuration, LAN and firewall configuration. This is particularly important in connection with any issues which may impact on your network security or local IT policies.


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