Call someone and start yelling something in a foreign language, and every few seconds repeat "I kill you."

Pretend to be the phone company and say this: Sir, as a customer of our phone company I am required to warn you: tomorrow we will be conducting a phone line blowup. Do you know what that is sir? No? Okay, have you ever noticed that sometimes a phone call is very noisy? That’s because your phone line is "dirty." Periodically, we "clean" these lines. Basically, we send a burst of electricity down all the phone lines. During this time, sir, make sure you don’t answer your phone no matter what happens, or else you will receive a shock of about 375 volts. Do you understand sir? Do not answer your phone if it rings, do not even use your phone. Due to the locations of all our phone lines, I cannot give you a definite time when we will be doing this. Just don’t use or answer your phone for all of tomorrow. Thank you sir. Sorry for the inconvenience.

Call someone and have a conversation like this:
Him: "Hello?"
You: "Yes?"
"Hello?"
"Yes sir. Can I help you?"
"No, uh, you called me."
"No, I’m sure you called me. What can I help you with?"
"No, you called me."
"Look, if you don’t tell me what you want, I’m gonna hang up."
Then hang up. A few minutes later, call the same guy again.
Him: "Hello?"
You: "You again? If you don’t stop calling me, I’m gonna have to call the police."

Place a call to someone’s parents while the child is out and say this: Sir? I’m from the northeast branch of the LAPD and your son has been arrested for shoplifting and car theft. Can you come down here?

Call Penthouse’s subscription department and tell them that you want to subscribe to Playboy, but you don’t know if it’s good. Then ask the guy to describe all the pictures in the latest issue.

Call Encyclopedia Brittanica and ask if they have "pocket size" editions of their encyclopedias.

Dial up a 7-11 in the middle of the night and ask the clerk if he's working there all by himself.Then ask what would happen if someone tried to rob the store. Then ask the clerk if he's afraid of death.

Call the police and explain that your dog's head is missing because someone broke into your backyard and killed your dog, then cut its head off.

Pretend to be a salesman selling souls.

(This one isn't a prank call) Get a complete stranger to do your answering machine message. For example, if you're a guy, get a girl to do the message.

Call your friend and when she answers, act as if you've been talking for 5 minutes already. When she asks you to start over say "Pay attention, damnit," and pretend to get angry.

Place a random phone call and ask to speak to yourself. For example, if your name is Joey, say "Can I speak with Joey?" When the person says that Joey isn't here, say "Joey? Hey, that's my name too!"