Using FaceTime http://support.apple.com/kb/ht4319
Troubleshooting FaceTime http://support.apple.com/kb/TS3367
The Complete Guide to FaceTime + iMessage: Setup, Use, and Troubleshooting
Troubleshooting FaceTime and iMessage activation
Using FaceTime and iMessage behind a firewall
iOS: About Messages
Set up iMessage
Setting Up Multiple iOS Devices for iMessage and Facetime
FaceTime and iMessage not accepting Apple ID password
Unable to use FaceTime and iMessage with my apple ID
You posted in the iPad forum instead of the iPhone forum. To get answers to your question, next time post in the proper forum. See https://discussions.apple.com/index.jspa I'll request that Apple relocate your post.
My wife and I have same iPhone 4, not 4s, model number A1332, same network and contracts, but she can FaceTime over 3G somehow and I can't. Don't understand as all these posts say it isn't possible. How is this?
Definitely over 3G as wifi is switched off, router off, and 3G symbol in corner of screen.
* FaceTime over a cellular data connection is available on the iPhone 4S or later, iPad (3rd generation or later, Cellular models) , or iPad mini (Cellular models). Find out if your carrier supports FaceTime over cellular.
I don't care if it's facetiming via carrier pidgeon. That's not a documented or supported feature of the iPhone 4. Apple explicitly states that it requires a 4S or higher. Asking why another 4 won't do it is pointless. We don't know why hers is diong it. The most likely explanation is that it's not actually a 4, but a 4S... but you'll deny that, so telling you to check is equally pointless.