Take it to apple. I had the same problem and they replaced it with a new phone. The problem is a "multi touch" problem.
Communication app freezes and random reboots are not related to the multi-touch function of the iPhone. These same app freezes have been reported with all of the visual/audio communication apps on most smart phones (IE: Phone app, Skype, Google Voice, FaceTime, etc.). From what I've understood, it has to do with network disconnection delay and the signal the phones get when this process is taking place...if for whatever reason the signal that tells the phone to disconnect the connection gets corrupted or fails before the actual connection stops, it can freeze up, causing the phone to become unresponsive.
I have had this happen during phone calls...I very rarely use FaceTime so I'm not a good judge on whether it's relevant to that too (but from OP's post, it must be). It's not a software issue though, because it is reported across the spectrum of smart phone devices from Samsung, Apple, HTC, and the like. I think it's a carrier network communication problem with the devices themselves...but we'll never get the specifics on that because the carriers don't want to put their dirty laundry out there.
The random device restarts could be one of a thousand different things...identifying a specific cause of that online would be like trying to figure out why a person sneezed a week after it happened.