It should work.
You need to go to Settings >> FaceTime >> Use Mobile Data and set it to on.
The person you are calling must be capable of receiving the call, so either on iOS 6 or on wifi.
If it still doesn't work, check with your carrier to make sure they allow it.
Edit: DUH! Sorry, I didn't register that you're on an iPad2. You need to be on 3rd Gen for FaceTime over 3G.
Using FaceTime http://support.apple.com/kb/ht4319
Troubleshooting FaceTime http://support.apple.com/kb/TS3367
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