Some guy has a a whole rationale on why just diverting telemarketers isn't good enough -- we have to strike back. After you've read about this guy's personal vendetta check out this funny Flash cartoon of the device in action. (keep in mind that the telemarketer is speaking to a set of prerecorded wave files).
By the way, I find the most effective way to deal with telemarketers is to simply say nothing and scratch yourself with the phone. You'll kill two birds with one stone!