Bluetooth phone call volume too low
Had this problem with my 7+; upgraded to an iPhone XS Max running 12.4 (now) and still have the same problem:
Phone volume on Bluetooth is super low. When I play music through the same bluetooth device,I have no issues. When I go to Settings | Volume Limit and play with the volume, sometimes it fixes it in real time, but the problem always comes back.
iPhone XS