Kevin Drum, in commenting on this entry about zapping the TeleZapper, asked a very reasonable question:
Sarcasm aside, why do they want to defeat devices like the TeleZapper? Surely anyone willing to spend $40 and expend the effort to install such a thing is pissed off enough about telemarketing calls that they are genuinely unlikely to respond to a call. Why would you want to contact such a person?
This, I think, has to do with an old door-to-door salesman's rule of thumb, namely, that the best doors to knock on were those where there were signs saying "No Solicitors." These signs were mostly (at least in the salesmen's worldview) put up by people whose "sales resistance" was nil, people who knew (or whose spouse, perhaps, knew) that any salesman who got them to open the door was pretty well guaranteed a sale. People who could easily growl, "get lost" to a salesman didn't need "No Solicitors" signs. So, anyone who goes to the trouble and expense of buying and installing a Telezapper is quite likely someone who has little to no sales resistance and is probably a pretty good mark.