Use the dial pad to make a call
The dial pad can be used like a typical phone to make a call to a phone number. (You can also type in names or groups.)
To dial a number from Teams, go to Calls 
and then enter the number of the person you want to reach by using the dial pad located on the left. Then click Call 
To call a person by name, type the name of a person into the dial pad. And to make group calls, enter multiple names and/or numbers, and click Call 
To access your dial pad during a call, go to your call controls and select Dial pad 
You can also check out the First things to know about calls in Microsoft Teams article for more calling options and features.
Important: You won't be able to make a 911 emergency call if Teams isn't connected to the internet.
Note: If you don't see the option to dial a number, your org may not have purchased a calling plan. But you can still make calls to other people in Teams.