In a sense, yes its free. It uses your minutes from your allowance. So for example, if you get 1200 minutes per month, you can use your entire 1200 minutes to make international calls and you wont be charged an extra penny. I found this service because my friends went to Afghanistan so i thought i would call them.
The only charge you get is about 10p when you send a text to register for the service, plus they pay you 2p per minute.
anyone else use this? im still waiting for them to send my personal number