Blue Box - Operation

Operation

The operation of a blue box is simple: First, the user places a long distance telephone call, usually to an 800 number or some other non-supervising phone number. For the most part, anything going beyond 50 miles would go over a trunk type susceptible to this technique.

When the call starts to ring, the caller uses the blue box to send a 2600 Hz tone (or 2600+2400 Hz on many international trunks followed by a 2400 Hz tone). The 2600 Hz is a supervisory signal, because it indicates the status of a trunk; on hook (tone) or off-hook (no tone). By playing this tone, you are convincing the far end of the connection that you've hung up and it should wait. When the tone stops, the trunk will go off-hook and on-hook (known as a supervision flash), making a "Ka-Cheep" noise, followed by silence. This is the far end of the connection signalling to the near end that it is now waiting for routing digits.

Once the far end sends the supervision flash, the user would use the blue box to dial a "Key Pulse" or "KP", the tone that starts a routing digit sequence, followed by either a telephone number or one of the numerous special codes that were used internally by the telephone company, then finished up with a "Start" or "ST" tone. At this point, the far end of the connection would route the call the way you told it, while the users end would think you were still ringing at the original number. KP1 is generally used for domestic dialing where KP2 would be for international calls.

The blue box consisted of a set of audio oscillators, a telephone keypad, an audio amplifier and speaker. Its use relied, like much of the telephone hacking methodology of the time, on the use of a constant tone of 2600 Hz to indicate an unused telephone line. A free long distance telephone call (such as the information operator from another area code) was made using a regular telephone, and when the line was connected, a 2600 Hz tone from the blue box was fed into the mouthpiece of the telephone, causing the operator to be disconnected and a free long distance line to be available to the blue box user. The keyboard was then used to place the desired call, using touch tone frequencies specific for telephone operators. These frequencies are different from the normal touch tone frequencies used by telephone subscribers, which is why the telephone keypad could not be used and the blue box was necessary.

Development and use of the blue box was largely enabled by Bell Telephone's policy of publishing all technical documentation regarding its equipment. In response to the development of this and other means of telephone hacking, the company began to develop other means of securing its system, without publicly disclosing the details. These included modifying telephone central offices to listen for the 2600 Hz tone coming from a subscriber telephone. This, plus the investigation and prosecution of several hackers by the FBI, finally made the blue box and other phreaking equipment obsolete. The hacking community evolved into other endeavors, however, and there currently exists a commercially published hacking magazine, titled 2600, a reference to the 2600 Hz tone that was central to so much of telephone hacking. The blue box no longer works in North America primarily because the phone system has converted to digital, and (analog) inband signalling is no longer used.