Same here. Since I got a Nexus 6P Bluetooth calls are a terrible plain in my 2015 BMW 428i Gran Coupe. Used to work fine on my Moto X 2nd Gen. Has to be Marshmallow. I have updated the phone and the car's firmware.
I posted this on the BMW forum, Bimmerpost:
"The bluetooth on my 428i Gran Coupe always worked fine until I got the new Nexus 6P phone with Android Marshmallow (v. 6.x). Despite updating the phone and the BMW's firmware, I still have calls drop off my Blueooth (they stay on the phone) and the just as randomly a few second later, pick up on Bluetooth in the car.
This is a problem because I can't hear the call and have to switch to speaker on the phone. Then it goes back to Bluetooth.
The call time on the car will start again at :00 when the call goes back to BT, but it will have been on the phone the whole time.
Also, I can dial using the car's phone/BT controls, but can't hang up with them.
This problem is being discussed a lot on Android forums. Does anyone have any advice? Has anyone taken it up with BMW?
It's gotten so bad, I unpaired my phone from my car and just use speakerphone. Sucks.
Streaming music works fine, so it's just the phone."
Anyone have any prayer that Google is addressing this? It basically destroys a major feature of the phone's functionality.