I think what has happened is that anyone who gets the number gets inundated with cult of $cientology phone calls at any time of the day or night. While I was connected to them I got phone calls (not a lot but I did get them) at midnight, 2 a.m., and 4 a.m. The phone company has probably decided just not to issue the number for a while.
I doubt that's the case, especially if it's AT&T. I sometimes get telephone calls for individuals that had my phone number before I got it 13 years ago.