Yes, I thought that volume control might work both ways.
Bluetooth is a funny business; for many people it just sort of appeared on the scene without much fanfare. I remember when I was in the computer business, some customers presented me with a computer for maintenance: "And it has Bluetooth!". -At the time, I simply had no idea what it was, and they had no idea what it was for, however, it had been a major selling-point for the computer!
Searching the Internet, it seems that the detail of pairing phones and cars is a troublesome area, particularly transferring your contact list. It's a case of Car problem or Phone problem? Who are you going to call for a solution? Bluetooth is not well documented at the user level.