Bluetooth hands-free profile
Also called HFP · headset mode
The hands-free profile is the Bluetooth mode a headset switches to when its microphone is in use. It carries sound both ways at a much lower quality, which is why music can turn thin the moment a call starts.
In practice
The hands-free profile is why music turns thin and mono as soon as a call starts on a Bluetooth headset. It is how classic Bluetooth fits a microphone and listening audio into one link. Newer headsets with wideband speech sound better and LE Audio improves it further, but only when both ends support it.
How to check
Play music, then open the microphone test with the headset mic selected. If the music drops in quality the moment the mic opens, the headset has switched to hands-free mode. Using a different microphone keeps the headset in stereo mode and restores full quality for listening.