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 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 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 that 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 line at their disposal could open many more possibilities than just free phone calls: they could dial into phonecompany test switches, to long-distance route operators, and into conference lines--which meant they could set up their own phreaker conference calls. Quite simply, possession of a blue box gave the user the same control and access as a Bell operator. When operator-controlled dialing to Europe was introduced in 1963, phreakers with MF-ers found they could direct-dial across the Atlantic, something ordinary subscribers couldn't do until 1970.
The only real flaw with blue boxes was that Bell Telephone's accounts department might become suspicious of subscribers who seemed to spend a lot of time connected to the 1-800 numbers of, say, Holiday Inn or the army recruiting office and might begin monitoring the line. Phreaking, after all, was technically theft of service, and phreakers could be prosecuted under various state and federal laws.
To get around this, canny phreakers began to use public phone booths, preferably isolated ones. The phone company could hardly monitor every public telephone in the United States, and even when the accounts department realized that a particular pay phone had been used suspiciously, the phreaker would have long since disappeared.
By the late 1960s blue boxes had become smaller and more portable. The bulky vacuum tubes mounted on a metal chassis had been replaced by transistors in slim boxes only as large as their keypads. Some were built to look like cigarette packs or transistor radios. Cleverer ones--probably used by drug dealers or bookies--were actually working transistor radios that concealed the components of an operational blue box within their wiring.
What made Bell's technology particularly vulnerable was that almost anything musical could be used to reproduce the tone frequencies. Musical instruments such as flutes, horns, or organs could be made to re-create Bell's notes, which could then be taped, and a simple cassette player could serve as a primitive MF device. One of the easiest ways to make a free call was to record the tones for a desired number in the correct sequence onto a cassette tape, go to a phone, and play the tape back into the mouthpiece. To Bell's exasperation, some people could even make free phone calls just by whistling.
Joe Engressia, the original whistling phreaker, was blind, and was said to have been born with perfect pitch. As a child he became fascinated by phones: he liked to dial nonworking numbers around the country just to listen to the recording say, "This number is not in service." When he was eight, he was accidentally introduced to the theory of multifrequency tones, though he didn't realize it at the time. While listening to an out-of-service tape in Los Angeles, he began whistling and the phone went dead. He tried it again, and the same thing happened. Then he phoned his local office and reportedly said, "I'm Joe. I'm eight years old and I want to know why when I whistle this tune, the line clicks off."
The engineer told Joe about what was sometimes known as talk-off, a phenomenon that happened occasionally when one party to a conversation began whistling and accidentally hit a 2600-cycle tone. That could make the line think that the caller had hung up, and cause it to go dead. Joe didn't understand the explanation then, but within a few years he would probably know more about it than the engineer.
