When someone calls you, you'll get a notification that lets you accept or decline the call.
Select Accept with audio to answer with audio only.
Select Accept with video to accept the call and enable video as well.
Select Decline call to decline the call and go on with your day.
When a call comes in to Teams, a notification pops up, letting you know. Tap Answer (the blue one!) to start talking.
When you get an incoming call, you'll see an onscreen notification. Tap the notification to answer.