My BT Headset connects with A2DP profile per default which is fine for listening to music. But for video calls we need a mic.
Of course you can switch the bluetooth profile manually within pavucontrol, but even better is modifying /etc/pulse/default.pa by adding auto_switch=2 to the line
load-module module-bluetooth-policy auto_switch=2
Restart pulseaudio with pulseaudio -k