SIP ALG (Application Layer Gateway) is a feature found in most routers which inspects VoIP/SIP traffic as it passes through and attempts to modify the messages on-the-fly.
This is sometimes useful, but in a lot of situations SIP ALG can cause problems. Quite often, the SIP ALG is poorly implemented and causes VoIP messages to be modified in a harmful way or simply corrupts them. 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).
It is our experience that the SIP ALG function on modern firewalls can be highly subjective. As it is dependent on the make and model of firewall, and possibly also the network configuration, we cannot provide a singular answer as to whether or not it should be enabled or disabled.