If you have iPhone 4S and above or iPad 3 and above you can do FaceTime over cellular data. I'm not sure if the over has be the same comparability or if it doesn't matter.
I understand your concerns regarding FaceTime options! I personally love using FaceTime to stay in touch with my daughter (away in college). FaceTime over cellular network is available on your iPhone 4S (latest iOS required). Please find this information also at this http://support.apple.com/kb/ht1937 link. FaceTime utilizes data (no minutes used while using FaceTime).
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