why can't I use face time on iPad 2 with 3g iOS 6.0.1

Because FaceTime over 3G is not compatible with the iPad 2. An iPad (3rd generation) or later is needed for that. You will need Wi-Fi to place a FaceTime call.

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

The Complete Guide to FaceTime + iMessage: Setup, Use, and Troubleshooting

http://tinyurl.com/a7odey8

Troubleshooting FaceTime and iMessage activation

http://support.apple.com/kb/TS4268

Using FaceTime and iMessage behind a firewall

http://support.apple.com/kb/HT4245

iOS: About Messages

http://support.apple.com/kb/HT3529

Set up iMessage

http://www.apple.com/ca/ios/messages/

Troubleshooting Messages

http://support.apple.com/kb/TS2755

Setting Up Multiple iOS Devices for iMessage and Facetime

http://macmost.com/setting-up-multiple-ios-devices-for-messages-and-facetime.htm l

FaceTime and iMessage not accepting Apple ID password

http://www.ilounge.com/index.php/articles/comments/facetime-and-imessage-not-acc epting-apple-id-password/

Unable to use FaceTime and iMessage with my apple ID

https://discussions.apple.com/thread/4649373?tstart=90

 Cheers, Tom 😉