Participating in early access and feedback programs for new Teams Free features
If you are on a Teams Free application then the option to make phone calls is not available. Phone calls is only available with Teams for Business account and additional purchased Phone Calling licenses.
However, if you still have some remaining Skype Credit or an active Skype calling subscription then phone calls can be made from Skype Dial Pad site on the Web.
https://calling.web.skype.com/
Just be sure to sign in to this site with exactly the same account as you have used with Skype.