Video is turned on automatically when you join or are called from an unscheduled meeting in a channel. You can't change these video settings, but you can turn off your video once you're in the call if you don't need it.

                                                                                                                  

Call type

Video setting

Unscheduled meeting

Video on

Group or 1:1 call from a chat

Asks to join video

Scheduled meeting

Video off