2600 Hz is the frequency in hertz (cycles per second) that AT&T formerly put as a steady signal on any long-distance telephone line that was not currently in use. Prior to widespread use of out-of-band signaling, AT&T used in-band signaling, meaning that signals about telephone connections were transmitted on the same line as the voice conversations. Since no signal at all on a line could indicate a pause in a voice conversation, some other way was needed for the switch to determine which circuits were available. This was accomplished by placing 2600 Hz on idle trunks, and removing it when a call was ready to use the circuit.

Knowing this, certain people developed a way to use a whistle or other device (i.e. a blue box) to generate a 2600 Hz tone on a line that was already in use, indicating to the far end switch that the line was idle again. When the tone stopped, the switch assumed another call was being set up, and listened for dialed digits indicating how to connect the call. Interoffice signalling was accomplished using CCITT MF digits, not the DTMF which was introduced later for subscriber use under the name "touch-tone".

In order for blue boxing to work, the initial call must be made across an interoffice MF trunk, so local calls (those which originated and terminated on the same switch) wouldn't work. By placing a call to a distant toll-free number, interoffice trunks could be involved but no billing was incurred. Using a blue box during a toll-free call would allow the user to call anywhere for free. Cracking the phone system became a hobby for some who came to be known as phone phreaks. In the 1970s and 80s some trunks were modified to filter out SF tone arriving from a caller. Later developments made 2600 signalling rare; thus the old fraud method became impractical.

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. By dialing a telephone number, covering one of the whistle's two holes, and then blowing the whistle, it could fool the phone company into thinking the line was not being used while, in fact, the user was now free to make a call to any destination in the world. The phreaker Captain Crunch adopted his nickname from this whistle. 2600: The Hacker Quarterly got its name from the frequency itself.

Late in the 20th century, long-distance companies adopted Signaling System 7, which puts all channel signals on a separate signaling channel, making it more difficult to break into the phone system.