TeleZapper

[1] Telemarketing companies typically use predictive dialers to place many calls simultaneously.

On detecting a ring and answer of any phone on the line, the Telezapper will immediately play a sequence of Special information tones or "SIT": one of eight internationally standardized signals that indicate a call cannot be completed.

Typically the "Intercept" or "IC" SIT is used, indicating the number called has been disconnected or changed.

The device is powered by a CR2032 battery, which is not documented in the manual and is replaced by opening the unit.

A predictive dialer merely checks for three conditions: Some telemarketing firms have turned off the SIT tone detector altogether in response to the TeleZapper trick, rendering it wholly ineffective.