From the Calls page, you can make an immediate web or video call to another person. Calls are a feature familiar from another Microsoft service, Skype. Calls are different from online meetings in that they cannot be scheduled and are one-to-one.
To make a call, go to the "Calls" page in Teams, find the person you want to call and select either the video camera or phone icon to make the call.

An alternative way to make a teams call:

A teams call can also be initiated via a chat conversation. You can start a Teams call by selecting the conversation of the person you want to call from the chat windows and click on the phone button in the top right corner of Teams.

Teams calls use the same tools as Teams meetings.

 

Look this: Teams FAQ