Shoot. I just read the find print. You only get free service for 120 days and 300 minutes a week. After that they make you buy credit. Here's the find print and link to it:
Free* calls with Voip Stunt
Not only can you call your online friends for free, Voip Stunt also offers free* calls to any regular land-line in various popular destinations (see panel).
To counter misuse of our network we have limited these free* calls for our trial users. If you want to enjoy longer free* calls to these destinations, just top up your account by buying credit.
Voip Stunt Freedays
If you buy credit, you will get 120 Freedays (unless stated otherwise). This means you can call all countries in the free destinations list for 120 days at no costs. When the 120 days are over, you will keep your credit, and a minimal rate (for instance 1 cent/minute) will be charged for the free destinations (this rate depends on the country and will one of these days be displayed on the rates page).
http://www.voipstunt.com/en/free.html