caller id vs. the telemarketers

Caller ID has done a great job at reducing the volume of telemarketer calls we get. They all come up as unknown caller, so we don't pick up the phone until we hear a familiar voice on the answering machine.

My best friend has a device that sends a disconnect tone across the phone line whenever he picks up the phone. Telemarketer computers get the signal and mark the line down as inactive, purging it from the database. It seems to have no effect on other calls, though.