From the iPad User Manual:
On an iPad 2 or later, you can use FaceTime to make video calls to other iOS devices or computers that support FaceTime. The FaceTime camera lets you talk face-to-face; switch to the iSight camera on the back to share what you see around you.
Note: FaceTime may not be available in all areas. On iPad Wi-Fi + cellular models, FaceTime calls can be made over a cellular data connection. Cellular data charges may apply.
To use FaceTime, you need an Apple ID and a Wi-Fi connection to the Internet. When you open
FaceTime, you may be prompted to sign in using your Apple ID, or to create a new account.
Make a FaceTime call: Tap Contacts, choose a name, then tap the phone number or email address the person uses for FaceTime.
You can also make a FaceTime call from the Contacts app.
Using FaceTime http://support.apple.com/kb/ht4319
Troubleshooting FaceTime http://support.apple.com/kb/TS3367
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