In Phreaking, the green box was a device whose function was to manipulate the coin collection mechanism of payphones.
According to the text file "The Green Box"[1] by "The Blue Buccaneer" and "The Tracker", each of these frequencies should be sounded for at least 900 milliseconds.
A modified phone with a poor-quality diode could be quickly identified by the constant clicking sound produced by the coin return mechanism.
Children would gather in the area to collect coins after a caller left, often to buy candy at the store across the road.
More recent research (2001) on a Flowbox by "Flame0ut & PrussianSnow" on Nortel Millennium payphones in Canada[3] indicate that the Collect signal on many systems is a current reversal.