I got one of those $30 Tele-zappers, and put it on my phone line. It gives out that four-tone "this number is disconnected" signal whenever the phone connects, whether calling or answering. It's especially useful with computerized dialing systems that dial a whole bank of numbers blindly, since the false "disconnected" signal usually causes the computer to delist your number. Then when telemarketers sell or trade their lists, your number is propogated as "disconnected".
It's an extra layer of protection, along with the do-not-call list, which has some definitional holes in it.