someone has built what I asked for

The TeleZapper uses the technology of telemarketers’ automatic dialing equipment against them. When you or your answering machine picks up a call, the TeleZapper emits a special tone that “fools” the computer into thinking your number is disconnected. Instead of connecting you to a salesperson, the computer stores your number as diconnected in it’s database. Over time, as your number is removed from more and more databases, you’ll see a dramatic decrease in the number of annoying telemarketing calls you receive.