Don't answer your phone for the next thirty-five minutes (idea)
When I was a child, I was absolutely convinced that being ten years old meant that I was far too old to need a babysitter. So, I convinced my parents that I could be safely left home alone, that I knew all the emergency phone numbers, etc.
One of the caveats was that I was not allowed to make outgoing calls on the phone unless it was someone I knew, and I had to take messages. But I realized very quickly that my parents wouldn't be able to check on me. I played with my Legos, mostly--being the "good boy" that I was--and that was fun enough to assuage even the boredom of a ten-year-old with an overactive imagination, until about 9:00 P.M.
I had read a list of prank calls a while back, and got feeling mischievous. I dialed a local number at random, but all that happened was the answering machine picked up. The prank would work much better on someone who was home, but I was so eager to try the prank, I left a message anyway. I said, "Your phone has been wired. Do not pick up the phone for the next hour, or the operator will be electricuted as she makes the connection. TRUST ME. Do not open the door. They're watching you."
I hung up and laughed and laughed and laughed, but the prank wasn't complete! The whole point was that someone had to be there to answer, to be terrified by the warning, to be scared into...
hiding in (his or her) loft, armed with marbles and Light-Bright pegs, in case someone tried to break into the house.
Because after twenty-five minutes, I could call back to this scared person. And call back, and call back, and call back, until someone answered. And then, knowing full well that they were picking up a phone they thought to be wired for 220 Volts, I could take a deep breath, and exhale it all in my best "electrocuted operator" impression: a blood curdling
Aaaaauuuuuggghhhhhhh!!!!...and then hang up and pee my pants from laughing.
katallen, it wasn't really me calling--I've just tried to reconstruct the call as it might have happened from the other end. This prank call was a classic before caller ID.