I don't know if it's still the case, but a few years ago if your bluetooth headset had a microphone, it switched from 'A2DP' to 'HSP' which has trash audio qual

I don't know if it's still the case, but a few years ago if your bluetooth headset had a microphone, it switched from 'A2DP' to 'HSP' which has trash audio quality. And it would happen almost invisibly - there was no indication in the OS of what was going on. Maybe it's better now?