2600 cycles per second
In the original analog networks, short-distance telephone calls were completed by sending relatively high-power electrical signals through the wires to the end office, which then switched the call. For long-distance telephone calls, the Bell System used a selection of tones sent over the trunks to control the system. In addition to dialing instructions, the system also included a number of other tones that represented various commands or status. 2600 Hz, the key to early phone phreaking, was the frequency of the tone sent by the long-distance switch indicating that the user had gone on-hook (hung up the phone). This normally resulted in the remote switch also going on-hook, freeing the trunk for other uses. In order to make free lines easy to find, the 2600 Hz tone was continually played into free trunks. If the tone was sent manually by the local user into the phone line, it would trigger the remote switch to go on-hook, but critically, the local switch knew he was still off-hook because that was signaled electrically, not by the tone (which their local switch ignored). The system was now in an inconsistent state, leaving the local user connected to an operational long-distance trunk line. With further experimentation, the phone phreaks learned the rest of the signals needed to dial on the remote switch.
John Thomas Draper (born 1943), also known as Captain Crunch, Crunch or Crunchman (after Cap'n Crunch breakfast cereal mascot), is an American computer programmer and former phone phreak. He is a legendary figure within the computer programming world and the hacker and security community. While Draper was driving around in his Volkswagen Microbus to test a pirate radio transmitter he had built, he broadcast a telephone number to listeners as feedback to gauge his station's reception. A callback from Denny Teresi resulted in a meeting that caused him to delve into the world of the phone phreaks. Teresi and a large percentage of the phone phreaks were blind. Learning of his electronic capability, they wanted him to build a multifrequency tone generator (the "blue box") to gain easier entry into the AT&T system, which was controlled by tones. Then they would not have to use an organ and cassette recordings of tones to get free calls. A blind boy, Josef Carl Engressia, Jr., who had taken the moniker of Joybubbles had perfect pitch and was able to identify the exact frequencies. They informed him that a toy whistle packaged in boxes of Cap'n Crunch cereal could emit a tone at precisely 2600 hertz, the same frequency that was used by AT&T long lines to indicate that a trunk line was ready and available to route a new call. Experimenting with this whistle inspired Draper to build blue boxes: electronic devices capable of reproducing other tones used by the phone company. Teresi hosted a long-running radio show under the name "Dennis Terry", and operated an oldies record store in San Jose, CA. Draper has long maintained a nomadic lifestyle, as of May 2013 he resides in Las Vegas, Nevada. Engressia passed away on August 8, 2007 (aged 58).