Having studied the US telephone system, they realized that it used a series of musical tones to connect calls. They found they could imitate those tones, and steal free phone calls, by creating small musical devices called ‘blue boxes’ One famous ‘phreaker’. John Draper, even discovered that using a whistle given away inside a cereal box could do the same job as a blue box.
Question : A nature of the crime made free calls by copying ............. ?