http://www.voipstunt.com/en/index.html

New users can try VoipStunt out for free for a total of 60 minutes. During this trial period you can only call the destinations marked as free. Register your account by buying credit in order to extend your free calls.Registered users get max 300 minutes per week of free calls, measured over the last 7 days and per unique IP address. Unused free minutes cannot be taken to the following week(s). If limit is exceeded the normal rates apply. During your Freedays you can call all destinations listed as “Free” for free. When you have run out of Freedays, the normal rates apply. You can get new Freedays by buying credit.