Issues such as one way audio, lack of incoming calls, registration issues and etc. can be due to SIP ALG
To disable SIP ALG telnet into the router and enter the following command.
1. > sys sip_alg 0 — Disables sip alg
2. > sys commit — Apply changes
3. > sys reboot — Reboot router
Once router is back online, reboot the ip phone or press re-register.
To re-enable SIP ALG run the following command:
sys sip_alg 1 — enables sip alg
Update: SIP ALG is now disabled by default and can be enabled/disabled using the GUI interface under NAT >> ALG
