With the increased dependence on personal computers, the fear of "hackers" has almost become a national obsession. In all this attention, however, the ancestor to computer hacking -- phone phreaking -- has been mostly ignored. Like computer hacking, this is an activity wholly dependent on human error, luck, and a determination to explore forbidden knowledge. Several poor decisions on the part of the phone company, coupled with a few teenagers' chance discoveries, led to widespread exploitation of the long-distance telephone network's inherent weaknesses. This essay will explore how the phone system works, and how it can be manipulated. (1)


Normal telephone dialing originally used pulse, which is simply a series of electrical interruptions; each click rapidly breaks and reconnects a the local loop (a circuit formed between the ring and tip wires of the telephone, and the telco equipment in the local company office). (2) The clicks represent values from 0 to 9 (0 is ten clicks). Pulse was designed to move relays a certain distance, based on the number of voltage pulses received by the equipment. (Basic Telecom, part V) Although it is rarely used anymore, most phones still have a switch to allow pulse dialing. In fact, on any phone, pulse dialing can be simulated by quickly depressing the switchhook multiple times (once for "1", twice for "2", etc.). This is one way to bypass a dialing lock, which is intended to prevent anyone from making outgoing calls from a particular phone (often used in offices or other places where many people have access to the telephone). Another pulse dialing trick, called step crashing, allows a caller to break through a busy signal and interrupt a call. If the caller dials, for example, 255-4356 and gets a busy signal, he can hang up and dial 255-4355. Then, right after the last click of the last dialed digit, but before the ringing starts, he presses the switchhook. If this is done correctly, he will interrupt the conversation.

The new dialing system, still in use today, is called Dual Tone Multi-Frequency (DTMF), but is more commonly known as Touch-Tone. Touch-Tone dialing is better than pulse, because it is faster and can be used for signaling even after the call has been connected. (Basic Telecom, part V) DTMF also interacts more efficiently with modern-day, computerized switching equipment. Each button on a DTMF keypad produces a combination of two tones, one from the high group and one from the low group (see diagram).

1. The telephone system uses different technology now, and most of the procedures and devices described here can no longer be used. However, for the sake of clarity I will discuss them in the present tense.
2. Originally, when calls were manually connected, operators used pegs which had a ring and a tip of opposite polarities. These terms are still used to designate the two wires in telephone equipment. Tip is green, ring is usually red, and usually on the right-hand side of the telco junction box. Thus, the mnemonic "three R's" -- ring, red, right -- when working with the connections in a junction box.

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