I know this isn't really the coolest number I have come across, but it still counts as a fun number. I have a list of phone numbers I've stumbled upon. I have a number that when called, you are conferenced into a telephonic conference at IBM. I've called it several times and do they get annoyed and confused. I've got tons of numbers like this.
Anyway, I have a friend that works for Alltel. He gave me this number (620.386.9916) and referred to it as the Alltel ID Buster. If you call it, it will ask that you input a telephone number, if it is an Alltel customer, it will not ring their phone and will skip directly to their voicemail. He has mentioned Alltel has TONS of numbers like this that preform different functions, some administrative. I'll see if I can't get a list!