Obviously I can't get the number blocked because I don't know what it is, but they keep calling me very late at night to prank call me or whatever, and I actually found it quite funny the first 2 times, but now it's just annoying.
I would call AT&T. Just because it is blocked on your caller ID doesn't mean AT&T can't see it. AT&T should be able to help you out.
I never tried this, so I have no idea how it works, if it costs anything, or how reliable it works, but you can check TrapCall - Unmask Blocked Calls which supposedly will text you with a blocked callers number providing you follow their instructions.