iOS 14: call controls not displayed in lock screen
I found perfectly working WORKAROUND:
During Microsoft Teams meeting, just go to home screen, start dialing any number via built-in Phone app and stop even after 1 second. Your Teams meeting won't dissconnect, it will just pause. After that, when You go back to Teams and lock the screen, call controls will appear every time! Simple as that!!!
Of course You have to repeat those steps on every new Teams meeting.
I hope you find this guide useful.