National Marketing Classifieds - Wont stop calling!

Put the "Line Has Been Disconnected" tones on your answering machine befrore your greeting message. The short 3 tone signal tells the telemarketing computer the number has been disconnected. All calls are placed by computers and when they detect ringing signal they switch the outward call to the next available inward operator. The "disconnected" signal will remove the number from their computer so they don't waste time calling "dead" lines.
Read here