Telezapper

I saw a commercial for the Telezapper last night, and checked out their website today (http://www.telezapper.com). It’s a small device that you attach to your phone line. The claim is that it will automatically stop telemarketers by sending a signal to the telemarketers’ auto-dialer telling it that your line is out of service.

So is this for real? Does it work? What is this magic tone that tells an auto-dialer that the phone is disconnected, even though it was just answered (the telezapper sends its magic signal when you (or your answering machine) answer the phone)?

This question has been asked several times in recent months, so I will close this thread and direct any remaining unanswered questions to one of the earlier threads: http://boards.straightdope.com/sdmb/showthread.php?threadid=92161

bibliophage
moderator GQ