Introduction:
Many commercial routers and some firewalls implement SIP Application Layer Gateway (SIP ALG), and have the feature enabled by default. SIP ALG is a SIP Proxy on a NAT network device, and is intended to provide a method to bypass the traditional problems associated with SIP NAT traversal. Due to the nature of AltiGen's implementation of the SIP protocol, AltiGen provides it's own NAT traversal method. SIP ALG is nearly always incompatible with AltiGen IP phones, and should be disabled. Common symptoms of a SIP ALG causing a failure are one-way and no-way talk, as well as a display of "SIP REG FAIL..." on the display of the IP phone.

 
Often times router manufacturers will refer to SIP ALG as "SIP Aware," "SIP Transformation," or "SIP Support". Dealers should contact the vendor of the routing device to confirm the appropriate method of disabling SIP ALG.

Common Implementations:

SonicWall

SonicWall calls their SIP ALG "SIP Transformations

Note - for many SonicWall devices, you may need to enable "Consistent NAT", and increase the UDP Connection Time Out settings to a value like 240 seconds.

Cisco

Cisco refers thier SIP ALG as a "fixup"

Zyxel

Models: 660 family comes with SIP ALG enabed by default.

SpeedTouch

Models: ST530 v6 (firmware >= 5.4.0.13) comes with SIP ALG enabled by default.

FortiGate

Fortigate refers to their SIP ALG as "SIP Helper"