If you prefer to have your phone lock when activating the proximity sensor thus ensuring that the screen is completely off while on a call, we have added an option in Fongo 3.0+ to make that happen. Please note that when you enable this option, if you place your phone to your ear while on a call, the phone will lock and you will need to unlock it as you normally would.
To enable this option go to Account > Settings > Troubleshooting > Use Proximity Lock (check option)