I had iPhone 6 iOS 12.xx - it worked seamlessly with Vauxhall vector 06 car stereo bluetooth.
Upgraded to iPhone 13 iOS 15. It will connect a call to the car bluetooth, but now won't hang up. The phone hangs up, but the car stereo/bluetooth line stays as open as if the line is connected still. Only fix so far to pull stereo fuse from the car each time!
Apparently becoming a known issue with Apple at the moment
Read here that CarPlay issues with iOS 15 should be addressed "soon": h t t p s : / / w w w.macrumors.com/2021/09/30/ios-15-iphone-13-carplay-issues/
After ios15 iPhone 11 nad 13pro connects by Bluetooth to my Opel Astra but there is No sound when Im calling someone by Siri. On Every iPhone ios14 - it works great
Same happened to me. After ios15 Bluetooth calls are muted. Every iPhone with iOS 14 works fine.