The only place where you can make phone calls using your remaining Skype credit is the Skype Dial Pad site on the Web:
https://calling.web.skype.com/
If your credit has expired due to not being used with the last 180 days then you can always reactivate this credit on this credit reactivation site:
https://secure.skype.com/portal/settings/credit/reactivate
Just be sure to sign in to these sites with exactly the same account as you have used with Skype.