Make sure you have FaceTime and iMessage activated with your Apple Id on your phone. I had the same problem and went to the apple store today and that's how they recommended fixing it. As soon as I activated it on my phone it brought it in on my iPad (new one), but they said if it doesn't automatically recognize it you might have to sign out and back in with your apple id on your iPad.
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
iOS: About Messages
Set up iMessage
Setting Up Multiple iOS Devices for iMessage and Facetime