When you dial a phone number you use cell phone service to make the call and it has nothing to do with your phone being connected to WiFi or not connected. You must be connected to cell service to make a cell phone call. When you make a FaceTime it is WiFi over internet only and has nothing to do with cell service. This is why AT&T/Verizon don't support it because it would bypass their profitable voice minutes while "hoging" up data/internet usage.