Participating in early access and feedback programs for new Teams Free features
Remaining Skype Credit cannot be used to make phone calls directly from Teams Free application, but can be used to make calls 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.
If your Skype Credit has expired due to not being used within the last 180 days, this credit can be reactivated from this credit reactivation site: