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While you're on a call with another person (or a group call), you might decide you want to ask someone else a relevant question.
You can make another call while you're still on the first call. When the second call connects, you can merge it with your first call.
When the second call connects, go to the call controls and select More actions
> Call merge 
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Merge an incoming call with an existing call
While you're on a call in Teams, you might receive a call from another person (or a group call) and decide you want to merge the new call with the call you're already on.
When you accept the new call, your other call will automatically be put on hold. Then you can choose to merge the new call with your first call.
Go to the call controls and select More actions
> Call merge
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