When VoIP phones do not register accross IPSec VPN it is most likely the SIP ALG that is interfering with the traffic to and from the SIP server.
Unfortunately Sophos does not have an option to disable SIP ALG in the Web GUI
To disable SIP ALG you can follow these steps
Log into your Sophos device with SSH or web console
In the shell type in the following command
system system_modules sip unload
to confirm SIP ALG has been disabled type
system system_modules show
This setting does persist after reboots
Reboot the VoIP phone and it should now register with the SIP server
- Tags: SIP, site to site
