Call Distortion with all bluetooth devices with iPhone 12pro

With my iPhone 12 pro calls break up while using Bluetooth. I was able to determine that it's not my cell service since when the call begins to breakup I can swap from Bluetooth to speaker and the call is clear. At first I thought it was AirPods but realized it happens in car as well which also rules out location issues. Spent hours on the phone with apple support as well as in the store and it tests fine. Tried powering off and various resets and setting changes per support. So far nothing works. I've read other posts with the same complaint and the answer is to restart the phone but that does nothing. Streaming videos and music have no issues. Any suggestions welcome. TYIA