[mcw]

If you're not running a custom ROM, and you've done a hard reset - have you tried calling Sprint? Which hard-reset method did you use - and have you also tried reflashing the STOCK Sprint ROM (via executable)?

They may need to reset your account. If you have any other Sprint CDMA phone available, try using the online phone activation to switch your phone and see if the issue repeats.

You haven't setup any caller ID blocking or made any other radio changes to your phone (via PST or Phone Settings), right? I know, simple and basic things, but might help to know if others have any better answers.