SIP ALG (Session Initiation Protocol Application Layer Gateway) is a feature in routers and firewalls that inspects and modifies VoIP traffic to help it traverse firewalls and NAT devices, but can also cause issues with modern VoIP systems.

Many commercial routers implement SIP ALG (Application Layer Gateway)/SIP Helper, and have this feature enabled by default. While ALG could help in solving NAT related problems, the fact is that many routers' ALG implementations are wrong and break SIP.

An ALG understands the protocol used by the specific applications that it supports (in this case SIP) and does a protocol stateful packet inspection (SPI) of traffic through it. A NAT router with a built-in SIP ALG can re-write information within the SIP messages (SIP headers and SDP body) making signaling and audio traffic between the client behind NAT and the SIP endpoint possible.

Unless you know the SIP ALG on your router/firewall works (the SIP ALG on a Cisco router for example), we recommend that you disable it and all NAT traversal technologies including, but not limited to, SIP ALG (ALG), and SIP Stateful Packet Inspection (SPI), and SIP Transformations.

Symptoms of SIP ALG:

One-way audio, call/audio drops, internal transfer issues or the inability to make or receive calls. 

For more information on NAT and Firewall Traversal Recommendations