When someone calls you, you'll get a notification that lets you accept or decline the call.
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Click Accept with audio 
to answer with audio only.
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Click Accept with video 
to accept the call and enable video as well.
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Click Decline call 
to decline the call and go on with your day.
You can also view a Normandale video on how to make and take calls: How to Make a Teams Call (Video 5m 8s)