SIP ALG : SIP ALG (Application Layer Gateway) is a mechanism found in most routers that rewrites packets transmitted across the device. Certain protocols are processed by the application layer gateway (ALG) and rewritten to allow better flow through a firewall or when NAT (Network Address Translation) is employed. The SIP protocol is one of several protocols managed by this system.
One of the most common issues with VoIP solutions relates to audio transmission and presence of a firewall and/or NAT traversal being configured. In many cases, a properly configured system may still have audio issues when transmitting or receiving calls where only one party is heard during a call. Implementing the necessary changes to disable SIP ALG can oftentimes resolve these issues.
Here's how you can disable SIP ALG:
Navigate to MANAGE | VoIP
Click on VoIP
Enable the Check-box, Enable consistent NAT. NOTE:Consistent NAT enhances standard NAT policy to provide greater compatibility with peer-to-peer applications that require a consistent IP address to connect to, such as VoIP. Consistent NAT uses an MD5 hashing method to consistently assign the same mapped public IP address and UDP Port pair to each internal private IP address and port pair. CAUTION:Enabling Consistent NAT causes a slight decrease in overall security, because of the increased predictability of the address and port pairs.
Disable the check box, Enable SIP Transformations.
Optionally, Change the UDP timeout on the LAN to WAN Access rule.
Go to the Advanced tab of LAN to WAN Access rule, and change the UDP timeout from default 30 seconds to 120 seconds.