When I was a child, I was absolutely convinced that being eight 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 answer the phone unless it was someone I knew, so the answering machine was running with the volume turned all the way up, so I could hear if it was my parents calling to check on me. We got a few calls from telemarketers, and a few calls from friends of the family, but it was quiet enough to assuage even the paranoia of an eight-year-old with an overactive imagination, until about 9:00 P.M.
The phone rang as I was reading in the living room, with a voice that said "Your phone has been wired. Do not pick up the phone for the next hour, or the operator will be electrocuted as she makes the connection. TRUST ME. Do not open the door. They're watching you."
This scared me enough (and sent my imagination into enough loops) that I spent the rest of the night hiding in my loft, armed with marbles and Light-Bright pegs, in case someone tried to break into the house.

figured that out, thanks Jolph Very cute

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.

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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.

There used to be a rash of purported kidnappings that turned out to be hoaxes.

The plan was simple, the rich husband goes to work, and the wife stays home. At work the husband gets a call, "We have kidnapped your wife. Leave one thousand US Dollars on the corner of Hickory Street(or wherever) and leave. Do not call the cops, or she dies. Leave the money in under 40 minutes or she dies."

So the husband frantically calls home, only to get no answer(This was before answering machines). The trick was to call pretending to be the phone company, and saying something like "We're doing mantainence on the line, please don't pick up or use the phone for at least 2 hours" or "There's been an obscene caller in the neighborhood, and we're tracing him now. Please don't pick up your phone for the next hour, it'll probably be him"

Now with the husband convinced of the kidnapping, he takes the company money or bank cash and drops the money off. By the time the wife is found safe and sound, the money is already gone. Pretty smart, and almost no effort needed, short of 2 phone calls. Sorta like George Clooney's character in Out of Sight.

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There's another practical joke you can do on the secretary you hate. No, I haven't done this, but Jeff Foxworthy has, and I'm reciting from memory from his book:

You: "Yes, this is (phone company). We have a report of having wire difficulties with 555-2312"
Secretary: "Yes, maybe my boss reported it-"
You:" Well, we're going to send out a technician to fix the lines on the pole around 1 O'Clock. We ask that you not make any calls on this line, and if the phone rings Do Not pick it up, or it will send a burst of power over the lines. That could seriously injure the phone repairman."
Note: At our business, the phone board is setup to roll over, so if one secretary doesn't pick up after 4 rings, it rings at the next desk, for the other secretary down the hall to answer it.
So my accomplice looks around the corner, and dials the number at the right time with his cell phone.
Supposedly she "stared at the phone, no way she'll answer it. Then she realizes what will happen, gets up and starts running down the hall, screaming "Don't Answer the Phone! NOoo!"
Sure enough, somebody picked up, I, outside the open window, screamed "AAAArrrgh!"
Next thing, she's in the break room, lying down with an ice pack, saying "you killed the phone man!"