I've heard that people on iOS 6.0-6.1.4 & iOS 7.0-7.0.4 have been having problems with FaceTime and the only way to fix that is to update to the latest iOS update (which is iOS 7.1.1 for the 4S).
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yeah its just a duo chebag kind of way to get people to update to ios7. then they can try and brag on this at WWDC even though they won't explain that they forced users to update. very fake much like a lot of things in the cook era