How to diagnose Android phone that is not working properly via vehicle bluetooth?

I connected an Android phone to a vehicle via bluetooth to make/receive calls. Connecting the device via bluetooth was simple. Making calls, however, was a challenge. The vehicle reports that the Android phone is always without cell reception, and therefore will not place calls. Furthermore, the vehicle's *head unit* (aka *receiver*) shows a "no reception" indicator for the Android device's cell reception. Placing calls **from the Android device** through the vehicle's audio system works as expected. This also shows that the Android device has ample cell reception to make calls. **What techniques or apps can be used to diagnose what is going wrong?** Ideally, I would like to see what the Android device is transmitting to the vehicle that is causing the vehicle to think the Android device has no reception. For example, an app that simulates a vehicle's *head unit* (perhaps with added diagnostics) would be useful.