Thanks Karina for taking the time to answer this. This is really appreciated.
However the solution provided here is really useless : Turning off the microphone when trying to log into a skype call is not really an option.
When using the microphone, the bluetooth codec switches from AptX to SCO.
That turns off noise cancelling on my headset (sony WH-100X-M3). It actually turns on "ambiant sound mode taht amplify ambient sound...) - don't really know why.
I tried other headset (bose, parrot, logitech) with the same issue.
When using the bluetooth microphone, it brings back memory of bad landline quality sound from pre internet era.
This is definitely a macOS problem that's been present for ages. Windows / linux distribs and Andoid or iPhone devices don't have this issue.
Pre catalina, you could force the audio codec to AptX (or others) even when using the microphone but that doesn't seem to be an option anymore.
The bluetooth Explorer utility has been removed from xcode tools and only a really old version can run on my mac (m1). It lets me see which codec is in use (by going to Tools/Audio graph) but no way of changing that codec anymore.
The official apple solution of not using the bluetooth mic makes me really angry the way a solution to a broken lamp bulb would be to learn to live in dark would.
Come on apple, get your act together.
Thanks anyway for trying to provide some help.