I would suspect this is an issue from the phone's end. Maybe the volume drops automatically on calls? I remember on iPhones in the previous updates that there was a normal volume and separate ringer volume.
I went from a 2015 Ford Focus to a 2017 Jeep Renegade recently and am now having the issue with low call volume. I never have an issue in my husband’s Mazda or my old Focus. I’ve tried maneuvering the settings but no improvement.I would suspect this is an issue from the phone's end. Maybe the volume drops automatically on calls? I remember on iPhones in the previous updates that there was a normal volume and separate ringer volume.