I'm experiencing the same issue (poor quality on calls .. sound like I'm "far away", "in a closet or underwater", "muffled", etc.). Every so often, I'll have a good connection, but rare.
Yesterday, I also noticed that the Audio and Media Volume controls didn't respond as I'd expect. I'd end a call and the music would resume, but very loud .. the sliders for Audio and Media seemed to move in concert... until they didn't and then I could put them back to where they were.
Lastly, as I drove around, the volume would drop for a minute or two, then it'd resume as before ... and so on and so on. Anybody notice this? I vaguely remember something similar a few software versions ago, but I hadn't noticed the problem again until the last 2 OTAs.
[using iPhone 12ProMax, CarPlay, w/ and w/o Navigation on]