Great question! Yes, if you have the mobile app configured to make/receive calls when on your 3G or 4G network, it will use your phones data. The mobile app has settings that allow you to control when calls are made/received via wifi, or your 3g/4g data plan. If you need help configuring the app so that it only uses wifi, let us know. You'll want to make sure the 3G/4G switch is in the off position as shown if you wish not to use your phones data plan.