SIP ALG is enabled by default on many commercial routers and internet gateways (modems). It intends to prevent some of the problems caused by router firewalls by inspecting VoIP traffic (packets) and if necessary modifying it. SIP ALG modifies SIP packets in unexpected ways, corrupting them and making them unreadable. This can give you unexpected behavior, such as phones not registering and incoming calls failing.
Many routers have SIP ALG turned on by default. Some firewalls hide this setting and in some instances, it will require an upgrade of the firewall to change this.