Found this on a Facetime guid on the ne:
"Note that you will need normal cellular connectivity and the ability to send and receive text messages from your cellular phone number in order to complete FaceTime activation. If you do not have a cellular plan or SMS is not enabled for your account, you will not be able to enable FaceTime on your iPhone. In light of the addition of non-iPhone devices to the FaceTime network, and the eventual desire of users to continue using their iPhones on Wi-Fi networks after discontinuing their cellular service plans, it will be interesting to see whether Apple provides an option for registration of these devices—most likely using Apple IDs."
Hope this helps.