Several users have experienced an issue in which iPhone 4’s FaceTime feature does not work after a phone number is ported from Sprint, Verizon or T-Mobile to AT&T. Users experiencing this problem may receive the error message “(name of user) not available for FaceTime.”
This issue can generally be resolved by restoring, but not from a backup. It appears that bad holdover data from iPhone backups can cause the problem. Restoring as a new phone will delete contacts and other data, but may resolve this problem.
To do so, first backup your iPhone in iTunes. Next, connect your iPhone or iPod touch to your computer, click “Restore” in iTunes, then choose “setup as new phone.” You can later re-sync your apps and other information.