Hey DrPositano,
If we understand, call audio is not coming through your car speakers, but music plays as expected. We want to help with this. To start, are you able to choose for the audio to go to your speakers and it work as expected? Just in case, those steps are found under "Adjust the audio during a call" here: While on a call on iPhone
If not, make sure your iPhone is fully updated: Update your iPhone, iPad, or iPod touch
We also recommend reaching out to the stereo manufacturer to ensure there are no firmware updates available for your model. If no firmware or software updates are available, let's forget the device by tapping Settings > Bluetooth, then locate the name of the device and tap the 'i', then "Forget This Device". Restart your iPhone, then pair it again with your vehicle to see if the issue continues.
Let us know how it goes.
Cheers!