I have been successfully using my Motorola Moto G3, 2015, with Android Marshmallow 6.0, to connect the phone to the audio car (Toyota) system for months.
A few days ago, I received a whatsapp call (media audio) while the phone was connected via bluetooth to the car, and I successfully used the audio car system for the whatsapp call.
Since then, unfortunately, whenever I connect the phone via Bluetooth to the audio car system, only media audio (e.g. youtube videos, mp3, music, whatsapp calls, etc.) is transmitted, whereas calling another telephone I do not hear anything via car stereo. The phone works because if I disconnect the Bluetooth, the call is active and I can use the phone to speak/hear.
I tried to remove the Bluetooth phone device and add it again, and to disable transmission of media audio via Bluetooth to the car system, but the problem does not disappear.