If you move out of a WiFi zone while you are on a Fongo call, your device loses its network connection and the call drops. Your device will automatically switch to 3G/4G and you can make or take calls again. If you know you will be moving out of WiFi during the call, turn off WiFi in your device settings before the call starts. This way, you will start your Fongo call on 3G/4G and the call will continue on 3G/4G as you move out of the WiFi area.
If you are on a Fongo call on 3G/4G and move to into a known WiFi network (one that you've logged into before and your device will automatically connect to) with an Apple device, the call will drop. With an Android device, your call will remain active and the device will switch to the known WiFi network once your Fongo call ends.