* Is the call 1:1 or extremely small? If so, it's down to the preferences of the people in the call. Otherwise, for larger calls:
* Are you in a quiet environment?
* That was a trick question. You are not in a quiet environment. You think you are, because your human brain is good at filtering out background noise. Your microphone is not. You are not being forced to actually listen to what your microphone hears. The rest of us are. Mute your fucking microphone!
Sounds like a horrible policy though.
I can vacuum my house while on my bluetooth headset and people won't hear it on the other end.
I leave my headset unmuted, and even if my kids are shouting in the next room my coworkers can't hear.
Also, cardiod mics and good software filter out background noise.