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.
Networking and VOIP notes 
How you have been detecting SDP and other various headers? Wireshark? I have my telephone system located inside DMZ on these router and problems has not disappeared.
Thanks it helped me to work out situation that *97 and *20 doesn’t work. Once I put this port trigger in for port 5060, it works. Also, I have one way audio problem too, so I have to go to Security->Filter to disable “Block Anonymous Internet Requests”