2600 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 phone company to know when a line was free for use. So AT&T put a steady 2600 hertz signal on all free lines. Knowing this, certain people developed a way to use a whistle or other device to generate a 2600 hertz tone on a line that was already in use, making it possible to call anywhere in the world on the line without anyone being charged. Cracking the phone system became a hobby for some in the mostly under-20 set who came to be known as phreak s.
In the 1960s, a breakfast cereal named Captain Crunch included a free premium: a small whistle that generated a 2600 hertz signal. By dialing a number and then blowing the whistle, you could fool the phone company into thinking the line was not being used while, in fact, you were now free to make a call to any destination in the world.
Today, long-distance companies use Signaling System 7, which puts all channel signals on a separate signaling channel, making it more difficult to break into the phone system.