How to fix iMessage
If what is posted above does not resolve and issue persists:
Follow instructions here:
http://support.apple.com/kb/TS4268?viewlocale=en_US and if persists below should solve your iMessage issues.
1. Sign out and Turn off BOTH iMessage and Facetime (both use same device token)
2. Backup iTunes or iCloud (either one)
3. Settings -> General -> Reset -> Erase all Content and Settings
4. On the device complete iOS setup and Setup as New (skipping all the AppleID and iCloud parts to sign in)
5. Settings -> Messages and sign in iMessage
6. Send a "test message" to self using your own AppleID email address
7. Send a "test message" to self using your own phone number
8. If resolved restore from iCloud or iTunes backup
Facetime and iMessage use both a Server Token and Device token. Erasing device and restoring from backup resets the device token and resetting your appleID password resets the server token.
If having issues with iMessage and Sprint call them and ensure they have the correct ICCID provisioned that matches your iPhone. Also, if you have Sprint and integrated Google Voice then yes that may cause iMessage issues and you may only be able send iMessage using your AppleID, not with your phone number.