FaceTime

I'm still trying to figure out how FaceTime works, especially whether it uses your mobile talktime. Does Apple maintain a server with all FaceTime devices, like instant messaging programs such as MSN Messenger?
I think there is a central registry, which stores FaceTime devices by phone number:

• You can't FaceTime call a contact without a phone number
• When I turned off WiFi on my iPhone and tried to FaceTime call it, it didn't even ring. The calling phone kept trying to connect. This showed that the calling phone knew my phone wasn't on WiFi. If it dialled a normal call then negotiated FaceTime, my phone would still ring.

Things to try:

• Make a FaceTime call to a landline