Phone keeps switching to bluetooth during a call.

I'll start by saying that the response "Turn off your bluetooth during your call" is not helpful!  

I will take a call on my iphone 12 (all updated and restarted), and after a minute or two it will switch to one of my currently connected bluetooth devices.  It can switch to my Airpods, a third-party headset, or sometimes it will connect to my daughter's car stereo if she starts her car while I am on the call (and I'm not in the garage).  I have to continually return the call to my phone as the audio source.  How can this be prevented? 

P.S. this question has been asked before, but Apple prevents new replies being made to old questions, even though it never originally received an answer.