Answer a call in Microsoft Teams
When someone calls you, you'll get a notification that lets you accept or decline the call.
                     
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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 
When you get an incoming call, you’ll see an onscreen notification. Tap the notification to answer.