SIP ALG (Application Layer Gateway) is a feature found in most routers which inspect 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.
Guidance on disabling SIP ALG on popular routers:
To be added: