If you're using Sprint and Google Voice, see this support article from Apple: http://support.apple.com/kb/TS4091
Integrating your Sprint account with Google Voice may prevent your mobile phone number from being registered for use with FaceTime video calls and iMessage text messages.
This occurs only when your Sprint phone number and Google Voice number are not the same. It should not affect your ability to send or receive SMS or MMS messages, or to receive or place FaceTime calls or iMessages using an email address instead of your phone number.
The support article doesn't give a solution, but I infer that you would need to change something with respect to Google Voice. I don't have a Sprint account and I don't know what it means to integrate a Sprint account with Google Voice.