Application Layer Gateway (ALG) is a method allowing certain media streams (e.g. VOIP, some video streams) to get across the firewall.
ALG understands the protocol used by a specific application such as SIP. A NAT router with a built-in SIP ALG can re-write information within the SIP messages (SIP headers and SDP body) making signalling and audio traffic between the client behind NAT and the SIP endpoint possible.
SIP ALG is used to change IP address to WAN IP addresses in SIP packet’s SDP content.
For more information refer to the article at: http://en.wikipedia.org/wiki/Application-level_gateway