HFP (Hands-Free Profile)
The Bluetooth profile for two-way phone calls, designed for car kits and the reason headphones start sounding like a tin can the moment the mic turns on.
Where A2DP sends music in one direction, HFP sends speech in both, over a synchronous SCO link with fixed bandwidth. Originally that meant an 8 kHz sample rate and the CVSD codec — landline quality, in other words. HFP 1.6 in 2011 added wideband speech with mSBC at 16 kHz, sold as HD Voice, and later revisions added LC3 for an even wider band. It took over from the simpler Headset Profile and added what a car needs: caller ID, voice dialling, battery level.
Classic Bluetooth can't run A2DP and HFP at the same time on one link. Join a video call on wireless earbuds and you choose between good sound with no mic, or a mic with bad sound. Plenty of people quietly use the laptop's built-in microphone and keep A2DP.