I recently started having this problem. With Bluetooth activated, try opening your Bluetooth settings, go to the car, see if "phone audio" is checked. Not only was mine not checked, it wouldn't let me check it! 2 suggestions I researched have worked for me. First, leaving Bluetooth on all the time (not ideal). Second, clicking your data off, then back on again. The easiest way to do this which successfully turns phone audio back on is by pulling the widgets or notifications bar and toggling on and off "airplane mode". Can't say these techniques will work for you, but they work for me. I was extremely frustrated because I use Bluetooth daily in my car and have no explanation for why phone audio in my car stopped working. Good luck.