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A blue box is an electronic device created to defeat long distance call charges on the United States telephone network. It operates by playing a tone and the multifrequency tones that were used by analog long lines. The 2600 Hz was known as a supervisory signal, because it indicated the status of a trunk; on hook (tone) or off-hook (no tone). The below MF tones were used to send the switching codes over the trunk line. After calling a long distance toll free line the 2600 Hz tone would be pulsed onto the open line dropping the receiving end of the node but leaving the trunk line open and waiting for a new routing signal. The routing signal could be in the form of a regular telephone number with KP being the pulse to start the code and ST to end the code:

The tone pairs needed to remain on for 60 ms. With 60 ms of silence between digits. The 'KP' tone had to be sent for 100 ms. On the line there were 2600 Hz traps that could detect pure 2600 Hz signals that would alert the " MOTHER" technicians that someone was inserting their own tone on the line. To defeat that detection device one sent along with the 2600 Hz some pink noise (most of the energy on this signal should be above 3000 Hz). This signal would not make it over the toll network , but should have carried the call as far as the local toll center ). The addition of this pink noise was so that the traps won't find 'pure' 2600 Hz on the trunk. This was not a perfectly safe way to box, but it was meant to slow down discovery so the phreaker could make a few calls from a payphone before being detected and had to move along.

If one had gotten onto a an operator trunk line by putting in the following codes (the XXX = area code) they would get the following types of operators:

If you knew the jargon (there were manuals available), you could use these operators to help you obtain the necessary information and complete your calls without the local company being aware that you were using the line. See also social engineering.

The Blue Box was used as a phreaking tool to gain access to long distance switching systems, and thereby make free phone calls. Blue boxes no longer work in most western nations, as the switching system has changed to a fully-digital one which no longer uses special tones to signal status. For some time, it was a federal crime to use or possess blue boxes -- many phreaks would record the tones on a miniature audio cassette recorder and take the recorder to a pay telephone to prevent the FBI from tracking their illegal call usage. Often if the phreaks were caught they would be offered jobs working for Ma Bell rather than spending time in a federal prison as the phreaks often knew more about how the long lines worked than the engineers who built them.

In the early 1970sMillennia: 1st millennium 2nd millennium 3rd millennium Centuries: 19th century 20th century 21st century Decades: 1920s 1930s 1940s 1950s 1960s 1970s 1980s 1990s 2000s 2010s 2020s Years: 1970 1971 1972 1973 1974 1975 1976 1977 1978 1979 Events and trends when the concept was first being explored, a number of people made and sold blue, red and black boxA black box is a system whose input and output characteristics are well understood, but one has no idea what is going on inside''. There are several main senses: In electronics, a sealed piece of replaceable equipment—see line-replaceable unit . In computes such as John Draper also known as Captain Crunch. There was an article in Esquire Magazine that made him famous and put phone phreaking on the map. Others who made and sold blue boxes were Steve Wozniak (who learned the art from Captain Crunch) and Steve Jobs, who would later start Apple Computer (with Wozniak).