When a coin is put into a payphone, the payphone
emits tones to the ACTS (Automated Coin Toll System). A red box can fool the
ACTS into believing that an individual actually put money into the phone simply
by playing the ACTS tones into the telephone microphone. After ACTS hears the
simulated tones, an individual can place a telephone call for free. This sort
of action mimics what phreakers
did to fool the phone system into letting them make calls for free.
The Tech FAQ. What is Red Box? [Online, 2004.] The Tech FAQ Website.