trivia question

A friend, (who wished to remain anonymous :) asks: Perhaps you can forward for me a trivia question I've been trying to get answered. I'm pretty sure the phreak magazine "2600" is named after 2.6KHz, which I think was a tone used for some phone billing system that got cracked (perhaps Cap'n Crunch's whistle frequency?) The trivia question is this: can anyone point me to a definitive reference for what 2600 really was? An ACM article or the particular phone system or the cracker who exploited it would suffice. thanks!