Maybe some phones will use the phone's cellular connection for voice phone calls. If you have enabled wifi calling on your phone, if the vehicle has a wifi hotspot and you have purchased a data plan such as the OnStar $20/month unlimited plan, then I believe the phone will use the vehicle's wifi and LTE service before it will use the phone's cellular connection.You could mount the phone higher up, but in general the newer the phone, the smaller the antenna, and the less power it can use. That's how they keep adding junk to the phone without sucking the battery down. If streaming is important to you, and you can afford the $10/month, using the car's cell phone antenna will definitely be better for data. But when you make calls with the phone it's still going to use it's own cell signal, not that of the car.