Back then, 555 - 1212 numbers were free. When an information operator answered, the off-hook signal would NOT get passed back, thus the billing equipment wouldn't register as a charged call.

When the information number is used with a blue box, which "Diverted" the call, that nasty off-hook would "register". This immediately registered as a bogus call, and the billing computer printed out this anomaly.

It didn't take Ma bell security long to figure out what was going on, so they tapped the line because the false registering of completed information calls was ruled as "just cause" for a wiretap.

This person's arrest caused an uproar among the blind phreaks who attempted and succeeded in contacting this individual who independently discovered this fluke but didn't know how to recognize the Sup condition (Which was hard to do).

This person, as revenge against the phone company, contacted an independent writer Ron Rosenbaum who wrote a very controversial article for the October 1971 issue of Esquire called "The Secret of the little blue box", and blew the lid off of ATT's "Secret", which was actually blatantly printed in the Bell System Technical Journal".

In fact, an old ATT TV commercial was made that bragged about how fast "Multi-frequency" technology can switch calls, and how these lines were used for BOTH signaling and voice. This dude discovered accidentally how to clear a line using 2600 and put two and two together. He didn't understand supervisory signals or other internal codes. Thus, a very revealing article about the secret internal workings of Ma Bell was available for everyone to see.

At first, lots of college students figured this out, as they had direct access to the Technical Journals. Then, as more and more people got busted, more and more figured it out.