You can receive incoming calls either in the app or you can have them forwarded to your real phone number.
You manage these settings in the app under More > Call settings.
If incoming Internet call is enabled, you will receive the calls within the application.
If for some reason you do not have a good data connection, Internet call will fail.
You can choose to have your calls forwarded to your real phone number instead. In this case you do not need a data connection (such as WiFi or 3G/4G/LTE).
If you enable both Internet call and Call forwarding we will try reaching you first by Internet call and if that fails the call will be forwarded to your real phone number.