The https://calling.web.skype.com/ site is the only site you can make phone calls from using the remaining credit. If you receive the message saying that you don't have any paid products then either you are signed in to this site with the wrong account or you credit was deactivated due to not being used within the last 180 days.
If the last is the case, then you can reactivate your credit from this reactivation site:
https://secure.skype.com/portal/settings/credit/reactivate
To make sure that you are using the right Skype account, compare the Skype Name listed on your Skype Profile page with Skype Name referred to in the email you have received from Skype.