I have come across a number Linksys BEFSR41 routers that have a SIP ALG enabled on them. I have not been able to narrow this down to a specific firmware. When looking at the admin interface it doesn’t specifically state anything about the ALG feature.  When looking a packet capture however, the SDP has clearly been modified as well as various headers.

When the SDP is modified this could cause one way audio issues, especially if your provider utilizes a Session Border Controller (it’s a type of VOIP firewall) that handles NAT traversal.  When the headers are modified, call control (making and receiving calls) could be affected.

The SIP ALG can be bypassed with the below steps :

1. Log into the web interface of the Linksys.

2. Click on “Applications and Gaming”

3.  Click on “Port Triggering”

4. In the “Application” field type “SIP”.

5. Set the “Start Port” and “End Port” fields to “5060”.

6. If your service provider allows a secondary registration port, create that as a new application. As you can see above I am using port “5058” .

7. Reboot your SIP phones.