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             ³ Fun things to do with phones (and quite harmless) ³
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Next time you answer the phone, don't say "Hello?" -- say "Hi, can I speak to
  John please?"

When bored, call an operator asking for the nearest exorcism clinic - and make
  sure they know you need it NOW.

The converse of the first gag:  Call up someone, say "Hello?"  (Worked well
  when we had the *69 Callback feature here -- someone would call my store and
  hang up; I'd call 'em back and say "Mac's Computer Hut, CAN I HELP YOU?!?")

?11 (all but 911, of course) and ?00 are fun numbers to dial - over here, 200
  tells you what phone number you're calling from.

Call payphones at the mall to try and pick up women.

Call a computer store, asking for information on their New Model 34857 computer
  (or whatever number is the next logical step - TRS80 Model 5, Amiga 3000,
  Apple 4, Tandy 9000, TI 99/5, Commodore 512), and become irate due to their
  lack of knowledge on the new product.

(The above will work similarly with new cars and fast food chains)

Call McDonald's and complain about the quality of the Whopper you just bought.

Scream three-letter-government-agency-acronyms into the phone - you'd be
  suprised at what happens.