Blue box

A blue box is an electronic device that generates the in-band signaling tones formerly generated by telephone operator consoles to control telephone switches. Developed during the 1960s, blue boxes allowed private individuals to control long-distance call routing and to bypass the toll-collection mechanisms of telephone companies, enabling the user to place free long-distance telephone calls on national and international circuits.
At first, the use of these techniques was limited to a small group of "phreakers", which included, among others, Steve Wozniak. After the publication of "Secrets of the Blue Box" in October 1971's edition of Esquire, interest in the topic grew tremendously, both among end-users as well as the Bell System. The practice was ruled as telephone fraud by the Bell System and the courts and prosecuted vigorously.
Blue boxes worked because the telephone system used tones in the existing voice lines to send routing instructions, and these tones were not filtered out at the handsets. Subsequent telephone switching technologies used out-of-band signaling methods in the form of Common Channel Interoffice Signaling in a separate channel not accessible to the caller. Blue boxing stopped working as these systems were deployed.
A related device, dubbed black box, enabled the reception of calls without incurring a charge to the caller.


In November 1954, the Bell System Technical Journal published an article entitled "In-Band Single-Frequency Signaling", which described the process used for routing telephone calls over trunk lines with the then-current signaling system, R1. The article described the basics of the inter-office trunking system and the signals used to start, route, and end calls.
In November 1960, further technical details were disclosed in the Bell System Technical Journal in an article entitled Signaling Systems for Control of Telephone Switching. This article identified the specific single frequency and multi-frequency tones used to start and end calls, and to transmit the called number, on a long-distance connection.
This engineering design assumed that these signals would only originate in the automatic switching equipment. The designers were aware that the in-band signaling method was subject to false signals arising in the telephone handset from ambient sounds, and chose the frequency of 2600 Hz because it was not present in normal speech. This choice performed well in the normal use of telephones. It was not foreseen, that a telephone user could insert control signals into the switching system by sending the appropriate tones into the telephone handset.
Before the technical details were published, many users discovered unintentionally and to their annoyance that a 2600 Hz tone, used as a steady signal to mark currently unused long-distance telephone lines, or "trunk lines", would reset those lines. Joe Engressia, known as Joybubbles, accidentally discovered it at the age of seven by whistling. He and other famous phone phreaks, such as "Bill from New York" and "The Glitch", trained themselves to whistle 2600 Hz to reset a trunk line. They also learned how to route telephone calls by. At one point in the 1960s, packets of the Cap'n Crunch breakfast cereal included a free gift: a small whistle that, by coincidence, generated a 2600 Hz tone when one of the whistle's two holes was covered. The phreaker John Draper adopted his nickname "Captain Crunch" from this whistle.
The widespread ability to blue box, once limited to just a few isolated individuals exploring the telephone network, developed into a subculture. Famous phone phreaks such as John "Captain Crunch" Draper, Mark Bernay, and Al Bernay used blue boxes to explore the various 'hidden codes' that were not dialable with a standard telephone. Some of the more famous pranksters were Steve Wozniak and Steve Jobs, founders of Apple Computer. On one occasion Wozniak dialed Vatican City and identified himself as Henry Kissinger and asked to speak to the Pope. Wozniak said in 1986:
Jobs later told his biographer that if it hadn't been for Wozniak's blue boxes, "there wouldn't have been an Apple."
Blue boxing hit the mainstream media when an article by Ron Rosenbaum titled Secrets of the Little Blue Box was published in the October 1971 issue of Esquire magazine. Suddenly, many more people wanted to get into the phone phreaking culture spawned by the blue box, and it furthered the fame of Captain Crunch. Two major amateur radio magazines published articles on the telephone system in the mid-1970s. CQ Magazine published details on phone phreaking, including the tone frequencies and several working blue box schematics in 1974. The June 1975 issue of '73' featured an article describing the rudiments of the long-distance signaling network, how to construct red and blue boxes, and put them into operation. Around the same time, do-it-yourself kits were available to build one's own blue box.
In November 1988, the CCITT published recommendation Q.140 for the Signaling System No. 5, which caused a resurgence of blue boxing incidents in a new generation of users.
During the early 1990s, blue boxing became popular with the international warez scene, especially in Europe. The software was made to facilitate blue boxing using a computer to generate the signaling tones and play them into the phone. For the PC there were BlueBEEP, TLO, and others, and blue boxes for other platforms such as Amiga were available as well.


Local plain old telephone service works by watching the voltage on the telephone lines between the telephone company's exchange office and the customer's telephone. When the phone is on-hook the approximately 48 volt electricity from the exchange flows to the phone and is looped back without passing through the handset. When the user picks up the handset, the current has to flow through the speaker and microphone in it, causing the voltage to drop to under 10 V. This sudden drop in voltage signals the user has picked up the phone. This system works well for short-distance lines on the order of a few kilometers, but as the distance grows the capacitance of the wires begins to filter out the sharp changes in voltage. So while the system is fine for local connections to the exchange, it is not useful for watching the status of lines on long-distance connections between exchanges.
To address this need, the Bell System adopted a second system on the circuits that connected the exchanges. These lines were switched by a system known as a "tandem", which the local exchange would switch to when it recognized the number was not local, typically by dialing a "1" at the beginning of the number. The tandem included the routing systems and long-distance trunk lines needed to talk to tandems at other exchanges, thereby linking the exchanges together. The tandems also faced the problem that the DC signals did not work over long distances, so instead, they used tones played into the lines to indicate status and dial numbers.
The basic protocol worked by playing a 2600 Hz tone into the line whenever it was not being used. The tandems at both ends of a given trunk line did this. When a system received a call being routed to a remote system, it scanned the trunk lines between the two tandems looking for the tone. When it heard the tone on one of the lines, it knew it was free to use. They would then select that line and drop the 2600 Hz tone from their end. The remote tandem would hear the tone stop, drop their tone, and then play a supervision flash, making a "ka-cheep" sound, to indicate they had noticed the signal. The line was now free on both ends to place a call.
Dialing a POTS phone used the same voltage-drop system to indicate digits by rapidly cycling the hook, nine times to dial the digit 9 for instance. This was known as pulse dialing. As it also required rapid voltage changes, it too did not work over long-distance links. This is why long-distance calling required operator assistance well into the 20th century, long after local calling had been completely automated.
To address this problem and allow end-to-end user long-distance dialing, Bell introduced a second system that encoded digits as two tones, the multi-frequency signaling system, or "MF". Before the widespread use of end-user phones with touchpad dialing, the tone dialer was located in the tandem. When the user placed a long-distance call, the initial "1" connected the customer's line to the tandem, which then read the following digits, notably the area code, to find the target exchange. Once it had the target it scanned the trunk lines to that exchange looking for a free line, connected to it, and then relayed the rest of the phone number over the line using the tone dialing method. The remote tandem then decoded the tones and turned them back into pulses on the local exchange.
When the call was complete and one of the parties hung up the phone, that end of the connection would indicate this by playing the 2600 Hz tone again. The other end of the connection would hear the tone and cause their local call to hang up as well.


The operation of a blue box was simple: First, the user placed a long distance telephone call, often to a number that was in the target area. Usually, this initial call would be to a toll-free number or some other non-supervising telephone number. Using a toll-free number ensured that the phone being used for access would not be billed.
When the call began to ring, the caller would use the blue box to send a 2600 Hz tone. Hearing this tone, the remote office believes the user hung up, and disconnects the call on their exchange and begins playing 2600 to mark the line free. However, this does not disconnect the call locally, only physically hanging up the phone will do that. So, in this case, the user is left on a live line, one that is connected via a long-distance trunk line to a target exchange.
The user now stops playing the tone. The remote exchange interprets this loss of tone to mean the exchange's tandem is attempting to place another call. It responds by dropping its tone and then playing the flash to indicate it is ready to accept routing tones. Once the far end sends the supervision flash, the user uses the blue box to send 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 with an "Start" tone, "ST". At this point, the far end of the connection would route the call the way it was told, while the user's local exchange would presume the call was still ringing at the original number. There were two KP tones, KP1 would generally be used for domestic dialing, and KP2 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 methods 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 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 multi-frequency tones 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.


The ultimate "solution" to the blue box was already being developed when phreaking first became popular in the early 1970s, as Bell Telephone was prototyping the No. 4 Electronic Switching System. This system handled all call routing in a computer, sending routing instructions over separate dedicated data-only lines. This offered far better utilization of the expensive long-distance trunk lines.
For instance, if a local caller placed a call to a remote number using the tandem system, the call would have to be placed all the way to the remote number via the voice lines. If that customer was busy, the user had to listen for the busy signal and then hang up. This process required the trunk line to be connected for many seconds. Using electronic signaling, the connection request was routed over data lines, which could immediately send back a "line busy" response. This did not require the trunk line to be used at all. Although this might save only a few seconds, with millions of such calls being placed every day, the advantages quickly added up.
At the time, phreakers felt there was nothing Bell Telephone could do to stop blue boxing because it would require Bell to upgrade all their hardware. That was precisely what Bell was already planning to do, but as the network already included large numbers of existing switches that were susceptible to blue boxing, the switchover would take some time.
For the immediate term, Bell responded with a number of countermeasures. As existing electronic switching systems maintained logs of all calls made, including calls to toll-free telephone numbers, Bell began examining the logs looking for suspicious patterns of activity. For instance, lengthy repeated calls to information or national hotel reservation numbers might indicate the presence of a phreaker using a particular line. In this case, filters could be installed on those lines to block the blue box. Bell also would wiretap the affected lines. In one 1975 case, the Pacific Telephone Company targeted one defendant's line with the following equipment:
In the 1970s and 1980s, some legacy trunks were modified to filter out single frequency tones arriving from a caller.
The development of digital switching equipment and out-of-band signaling systems with separate bearer and signaling channels prevented the use of blue boxes. The "blue box" terminology has since been recycled for other purposes. The hacking community evolved into other endeavors and there currently exists a commercially published hacking magazine, titled , a reference to the 2600 Hz tone that was once central to so much of telephone hacking.

Frequencies and timings

Each multifrequency tone consists of two frequencies chosen from a set of six, shown in the table on the left. The Touch Tone encoding is shown by the table on the right:

1209 Hz1336 Hz1477 Hz1633 Hz
697 Hz123A
770 Hz456B
852 Hz789C
941 Hz*0#D

The rightmost column is not present on

consumer telephones.

Normally, the tone durations are on for 60ms, with 60ms of silence between digits. The 'KP' and 'KP2' tones are sent for 100ms. KP2 was used for dialing internal Bell System telephone numbers. However, actual frequency durations can vary depending on location, switch type, and the machine status.
This set of MF tones was originally devised for Bell System long-distance operators placing calls manually and predates the DTMF Touch-Tone system used by subscribers. The leading trunk prefix 1 was not dialed as the operator was already on a Long Lines trunk at this point.

Special codes

Some of the special codes a person could get onto are in the chart below. "NPA" is a telephone company term for 'area code'.
Many of these appear to have been originally three-digit codes, dialed without the leading area code, and the format of destination numbers dialed to the international senders has changed at various points as the ability to call additional nations was added.
Not all NPAs had all functions. As some NPAs contained multiple cities, an additional routing code was sometimes placed after the area code. For instance, 519+044+121 may reach the Windsor inward operator and 519+034+121 the London inward operator distant, but in the same area code.

Blue boxes in other countries

Another signaling system widely used on international circuits was CCITT Signaling System No. 4.
Technical definitions are specified in formerly CCITT Recommendations Q.120 to Q.139.
This was also an in-band system but, instead of using multifrequency signals for digits, it used four 35 ms pulses of tone, separated by 35 ms of silence, to represent digits in four-bit binary code, with 2400 Hz as a '0' and 2040 Hz as a '1'. The supervisory signals used the same two frequencies, but each supervisory signal started with both tones together followed, without a gap, by a long or short period of a single tone of 2400 Hz or 2040 Hz. Phreaks in Europe built System 4 blue boxes that generated these signals. Because System 4 was used only on international circuits, the use of these blue boxes was more specialized.
Typically, a phreak would gain access to international dialing at low or zero cost by some other means, make a dialed call to a country that was available via direct dialing, and then use the System 4 blue box to clear down the international connection and make a call to a destination that was available only via operator service. Thus, the System 4 blue box was used primarily as a way of setting up calls to hard-to-reach operator-only destinations.
A typical System 4 blue box had a keypad plus four buttons for the four supervisory signals. After some experimentation, nimble-fingered phreaks found that all they needed was two buttons, one for each frequency. With practice, it was possible to manually generate all the signals with sufficient timing precision, including the digit signals. This made it possible to make the blue box quite small.
A refinement added to some System 4 blue boxes was an anti-acknowledgment-echo guard tone. Because the connection between the telephone and the telephone network is two-wire, but the signaling on the international circuit operates on a four-wire basis, signal-acknowledgment tones tended to be reflected at the four-wire/two-wire conversion point. Although these reflected signals were relatively faint, they were sometimes loud enough for the digit-receiving circuits at the far end to treat them as the first bit of the next digit, messing up the phreak's transmitted digits.
What the improved blue box did was to continuously transmit a tone of some other frequency as a guard tone whenever it was not sending a System 4 signal. This guard tone drowned out the echoed acknowledgment signals so that only the blue box-transmitted digits were heard by the digit-receiving circuits at the far end.