When I'm about to call someone using teams, I always check wether they are in a call.
If they are in a call, I wait to call them. To check wether someone is in a call, I mouseover the 'status-dot' next to their userpicture.
It would be nice if the status-dot could be improved, so that I could see that someone is in a call without having to mouseover to see the info. For example by having a small phone icon next to the status-dot when their status is 'in a call'.