While blue boxes manipulated long distance switching, red boxes exploited payphone coin mechanisms by generating the exact audio frequencies that payphones used to register coin deposits. These devices replicated the distinctive sound signatures of nickels (66ms of 1700Hz and 2200Hz), dimes (33ms), and quarters (130ms) that payphones transmitted over the phone line to validate payment before connecting calls. The technique worked because early payphones used in band signaling, transmitting coin validation tones over the same audio channel as voice calls, making them vulnerable to audio replay attacks. Red boxes became popular in the 1980s and 1990s because they were simpler to build than blue boxes, requiring only basic tone generators that could be constructed from Radio Shack components or even programmed into early handheld devices. Phone companies eventually caught on and implemented cryptographic validation and out of band signaling that made red boxing obsolete, but not before thousands of payphones were essentially turned into free phones for anyone with a $20 handheld device.
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