Have you ever dialed a number and gotten that three-tone signal, "buup-boop-beep," of three increasingly higher frequencies, just before the voice says "I'm sorry. The number you have reached has been disconnected or is no longer a working number." Well, telemarketers use automated dialing computers to make their calls for them (you've probably noticed a moment of silence between the time you answer and the time they pick up). Most of their computers recognize that tone sequence and, upon hearing it, mark the number as "not working" in their data base. In fact, many of them recognize as little as the first tone.

So if you want to really annoy telemarketers, by removing your number from their data base, build a circuit to generate those tones. And if you build it with an 8052 dervative it will be back on-topic as well.

Joe

P.S. If you've seen those Radio Shack TeleZappers advertised here in the States, this is exactly what they do.