a red box is a phreaking device that generates tones to simulate inserting coins in pay phones, thus fooling the system into completing free calls. in the united states, a nickel is represented by one tone, a dime by two, and a quarter by a set of five. any device capable of playing back recorded sounds can potentially be used as a red box. commonly used devices include modified radio shack tone dialers, personal mp3 players, and audio-recording greeting cards.