I have 2 ipad's [both 4th generation]. 1 is wifi only and 1 has a cellular connection through Verizon. I have had nothing BUT TROUBLE tryin to utilize Facetime between these 2 ipads. When both ipads are set to wifi, I have no problem, when i turn off the wifi and only use cellular I cannot not achieve connectivity. They find each other, but will NOT connect. This was working prior to ios7 update. Apple engineering has been unwilling to return my calls.
if anyone has an iphone or a cellular ipad and a second ipad or iphone, I invite you to try this:
1. Make sure you have a good wifi connection and a good cellular connection.
2. On the cellular ipad and or iphone #1 - TURN OFF "Wifi" [Use cellular data only]
3. On the wifi ipad or iphone #2 [Turn cellular off on this iphone - use only wifi]
4. Now try and facetime between these 2 devices. Let me know if you succeed.