I've got an old TeleZapper that really works. You can get the same effect by simply recording the tones preceding that "...the number you are calling has been disconnected or is no longer in service ..." record the tones onto your answering machine message. Calls from an automated dialer listen for the tones indicating a bad phone number, and they automatically delete 'bad' phone numbers from their database.
BTW, some company invented technology to defeat TeleZapper.
I am about *this* close to deliberately screwing with telemarketers. Specifically, I want to ask them if ther mother knows they are a telemarketer, then reply "She must be really proud of you" and hang up.