Phone connected via bluetooth to car audio system: media audio works, calls don't

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.

Any help?

Thanks!