If you're having trouble using FaceTime, make sure the feature is turned on. Tap Settings on the Home screen and then tap FaceTime. Tap the On/Off button to turn it on or turn it Off and On.
You have to use the appropriate method for placing a FaceTime call, depending on the kind of device the person you're calling has. If you're calling an iPhone 4 user, you should use a phone number; if you're calling an iPad 2, an iPod touch, or a FaceTime for Mac user, you need to make the call using that person's e-mail address.
If you're on the receiving end of a FaceTime call, accepting the call is about as easy as it gets. When the call comes in, just tap the Accept button to take the call or tap the Decline button to reject it.

If this is not working, go to Settings > General > Reset > Reset all settings.
Then keep the Powerbutton and the Homebutton pressed at the same time until you see the Apple logo on your iPad screen.
I use Skype, because I think it is working better; http://itunes.apple.com/app/skype-for-ipad/id442012681?alreadyRedirected=1&mt=8
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