<< He sells his own telezapper but you can just download the SIT tone and record it to your answer machine! >>
The SIT tones that are on his site are the full, three-note "do-DO-DOOO!" tones you hear when you call a disconnected phone number. I imagine putting those on your answering machine would be pretty annoying.
I recently called a friend who had bought a genuine Telezapper and was surprised to hear only a tiny chirp when he answered the phone. I asked about it, and he said it was a Telezapper and that it had worked very well for him. The chirp was only about 1/10th of a second or so.
That's the advantage of buying a genuine Telezapper -- you don't have to play the full, annoying SIT tones every time you pick up the phone. You might be better off getting Caller ID and, whenever you get a call that is unidentified, play the MP3 of the SIT tones into the phone when you pick up. Predictive-dialing computers have a pause when you first answer to decide whether you are a person, an answering machine or a telco tone, so that might do the trick.
However, I've discovered the 99% solution to eliminating telemarketing calls: Call each of your credit card's customer service departments and ask them not to make telemarketing calls to you. I did that a few years ago, and my telemarketing calls went from 1-2 per day to 1 every month or two. Your credit card company can also "turn off" other types of marketing to you -- they can usually stop sending ads in your monthly bill, stop sending you those annoying credit checks, and stop mailing you marketing materials outside of your monthly statement. Since I turned off all the marketing my quality of life has increased dramatically. Try it, you'll never go back!