Voipstunt put some fineprint in the bottom of their homepage. It is even under the copyright description.
They set a limit of free calls. 300 minutes per week. It means 42 minutes per day. Not much.
I still remember there was no limit for it when I signup. I don’t know when they add this limit they have’t disclosed to the current users.
* Max 300 minutes per week of free calls, measured over the last 7 days. Unused free minutes cannot be taken to the following week(s). If limit is exceeded, a minimal rate of 1ct/min will be charged (as shown in the VoipStunt software).
The real situation is they do not use this limit to restrict the free calls.
I signin with my account. I added the minutes I used from July 1 to July 7. The total is 442 minutes. All free. No charges.
To confirm the result, I calculated the minutes from August 11 to August 17. I used 450 minutes. All free.
I still like it, Voipstunt.