First, you need to enable facetime for the first time by sending yourself a text message - anything will do. It only needs to happens once. It notifies your mobile network operator in some way. Then, you need to be on a cell phone call with your party. When you initiate the facetime call by tapping down on the button your phone will be talking to various systems, presumably at Apple, their service providers or at your MNO. One potential explanation can be found at http://youshottheinvisibleswordsman.co.uk/2010/06/25/iphone4-facetime-and-open-standards/ however it's still not clear how the receiving party's phone is receiving the invitation. One potential explanation is all iPhone 4 calls are registered on an SBC/Gateway of some sort, without your knowledge. A Wifi network trace during a cellphone call can confirm this hypothesis. If this hypothesis is proven true it can have serious implications in privacy and might uncover gaps in communication of terms and conditions during the iPhone 4 sales process.