there was an article in 'Poptronics' magazine about a couple of years ago about the SIT tones.
if you notice, the tones are the same as what you hear when you call a telephone number that is no longer in service.
i have been using them on my answering machine since the magazine article came out.
it took about two weeks for all my usual pests to quit bugging me.
they are the cheepest, surest way to eat dinner in peace.