“Draper heard about the whistle from other phreakers. The whistle easily played at 2600Hz, the perfect tone to, in Apple Inc. [co-founder Steve Wozniak’s words], “seize a phone line.”

Huh, I had always wondered why the hacking magazine was called 2600. Guess that explains it, neat!

Out of the Inner Circle covered this real well. It was a book printed in the early 80's