The application makes use of your data plan, so if you intend to use this then go careful if you are not on an unlimited plan. The VoIP app will use a 1 – 2KB data stream to establish the phone calls.
When making a call the Voxofon account will get charged for the international portion of the call. So that you are not left in the dark as to how much that will be, the Pre App will display a message with the call rate on screen when you are ready to dial.