SIP ALG (Application Layer Gateway) is a feature which is enabled by default in most Cisco routers running Cisco IOS software and inspects VoIP traffic as it passes through and modifies the messages on-the-fly.

In few situations this is useful, but in most situations SIP ALG can cause problems using the service. Very often, SIP ALG is poorly implemented causing VoIP messages to be modified in a harmful way or simply corrupts them. In such cases you may experience issues registering your handset, one-way or no audio, unable to receive calls, and issues with Busy Lamp Field (BLF) and Message Waiting Indicators (MWI).

For our Hosted VoIP service, including out Single User services, SIP ALG must be disabled on your Cisco router.

This article provides general guidance which should be applicable to nearly all Cisco routers running Cisco IOS, such as the Cisco ISR an ASR series routers.

1. Log onto the router's terminal (command line interface) via telnet, SSH or serial console
2. enable
3. configure terminal
4. no ip nat service sip udp port 5060
   For TCP also run no ip nat service sip tcp port 5060

Further information can be found at Cisco documentation.