There was a thread on this, and with a lot of posters discovering various things (that it only does it when you call or when you are called...I forget which one it was....if you mute the call and then unmute it, it clears up, and finally, if you turn of Hey Cortana, it fixes it). I had the same issue and turned off Hey Cortana, and no more complaints. It seems to be a bug in the OS or firmware that does not turn off Hey Cortana on certain calls, so it messes up the noise cancellation microphones. I am guessing it will be fixed, if it has not already been fixed with updates, but until I hear for sure. I have Hey Cortana disabled and phone quality issues were resolved.