Unfortunately, even when I do as you suggest, the next time I use the phone it goes back to being quiet. Since this is new behavior I'm hoping they will fix it so that it works as it did prior to the update.I had the same situation. What seems to have happened is that both the volume on the car and the phone are reset to lower levels after the software update. I used the volume control on the phone (iPhone 12 running IOS 14.3) and turned that to maximum and then adjusted the volume on the car as needed. Since then, all has been fine.
Turning down the volume during a call will only affect the phone volume. Turning down the volume while listening to music only affects music volume. The volume controls whatever is using the speaker at the time of adjustment.Yes, my phone volume is much higher than the music volume.