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.

In the 1960s, AT&T used in-band signaling, meaning that all signals regarding the management of telephone connections were transmitted on the same line as the voice conversations. Since silence 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 a 2600 Hz signal 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 device, known as 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. However, because the phone had never gone on-hook, the billing system would not be reset.

This technique would only take effect on an interoffice multi-frequency (MF) trunk, so it had no effect on local calls, which originated and terminated on the same switch. However, by placing a call to a non-local toll-free number, interoffice trunks could be involved without incurring any charge. Using a blue box would then close the toll-free connection and allow the user to dial any other number at the toll-free rate.

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 Captain Crunch adopted his nickname from this whistle. Others would utilise exotic birds such as canaries which are able to hit the 2600 Hz tone to the same effect.

In the 1970s and 80s some trunks were modified to filter out SF tone arriving from a caller. Later in the 20th century, long-distance companies adopted the out-of-band signaling system Signaling System 7. This system sends all control signals on a separate signaling channel, making it impossible to generate these signals from an ordinary phone line.