With chat, you can have private one-on-one or group conversations that are not in the public team channel. And with calls, you can make calls or check your history or voicemail.
View a chat
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Select Chat and select a one-on-one or group chat from the chat list.
Start a chat
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Select New chat
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Type the name of the person or people you want to contact.
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Select the down arrow to add a name to your chat group.
Make video and audio calls from a chat
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Select Video call
or Audio call 
to call someone directly from a chat.
Use Recent and Favorites
On the left side of the app, you'll see your Recent conversations as well as your Favorites.
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Hover over a conversation, select the three dots, and then select Favorite to keep your chat in your Favorites list.
Make calls and view history or voicemail
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Select Calls > Dial a number, and enter a phone number. When you're ready, select Call to start the call.
Note: If your organization has an Enterprise Voice license, you can call anyone from Teams even if they're not using it.
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Select Calls > History to see your call history.
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Select Calls > Voicemail to check your voicemail.
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If you don't have Calls, select Activity to see any missed calls and voicemail.
Source: Microsoft