In some locations, especially rural areas in the United States and Canada, pay phones were configured for "post-pay" operation.
Upon receiving the correct payment for the call, the pay phone would then connect the microphone and allow the caller to speak.
An artifact of this scheme was that the called party usually heard the ACTS tones produced by the pay phone upon coin deposit.
The clear box circumvented the handset mute by providing the phreak with another microphone, whose output is amplified and fed into an induction coil.
Several text files on the subject of the clear box observe that some calls, such as to time and weather services and other information lines, do not require voice input from the caller.