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payphones. It was used to convince operators a call was being paid for.¹

The blue box, however, was the most sophisticated of all. It put users directly in control of long-distance switching equipment. To avoid toll call charges, users of blue boxes would dial free numbers - out-of-area directory enquiries or commercial 1–800 numbers - then reroute the call by using the tones in the MF-er.

This is how it worked. Long-distance calls are first routed through a subscriber's own local telephone office. The first digits tell the office that the call is long-distance, and it is switched to an idle long-distance line. An idle line emits a constant 2600-cycle whistling tone, the signal that it is ready to receive a call. As the caller finishes dialling the desired number - called the address digits - the call is completed. All this takes place in the time it takes to punch in the number.

At the local office, billing begins when the long-distance call is answered and ends when the caller puts his receiver down. The act of hanging up is the signal to the local office that the call is completed. The local office then tells the line that it can process any other call by sending it the same 2600-cycle tone, and the line begins emitting the idle whistling tone again.

A phreaker made his free call by first accessing, say, the 1–800 number for Holiday Inn. His local office noted that it was processing a long-distance call, found an idly whistling line and marked the call down as routed to a free number. At that point, before Holiday Inn answered, the phreaker pressed a button on his MF-er, which reproduced Bell's 2600-cycle whistle. This signified to the trunk line that the Holiday Inn call had been completed - or that the caller had hung up prior to getting an answer - and it stood by to accept another call.

But at the local office end, no hanging-up signal had been received; hence the local office presumed the Holiday Inn call was still going through. The phreaker, still connected to a patiently whistling long-distance line, then punched in the address digits of any number he wanted to be connected to, while his local office assumed he was really making a free call.

Blue boxes could also link into forbidden areas in the Bell system. Users of MF-ers soon discovered that having a merrily whistling trunk

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