Hello Jolivier23,
If I'm understanding you right, your iPhone is automatically connecting to Bluetooth in your car when you're on a phone call when the car is started. Also, this occurs even when you have a headset connected to your iPhone. I can certainly understand why this would be a problem considering you aren't in the car when this happens. I'm happy to help you out with this.
It is expected for the Bluetooth connection to trigger the call to be routed to the car. This is designed for convenience since it makes it so that you don't have to manually connect to Bluetooth each time you're in your car. If you would like, you can adjust the settings that control how your calls are routed. To do this, go to Settings > Accessibility > Touch > Call Audio Routing. More than likely this is set to Automatic. There isn't a way to set it so that it automatically routes to a wired headset or the iPhone earpiece, however, setting it to Speaker might be helpful in your situation. This way you would not be temporarily disconnected from the person you're on the call with while you switch from the car Bluetooth back to your headset.
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