In other words, the box fooled the phone company into thinking no one had answered at the receiving end, and therefore billing was never started on the call. Participating in a "black box" call meant talking between rings, since the phone co. would continue to send rings (AC current) through the line which was heard as a loud buzzing noise. Few people build or use "Black Boxes" anymore, given that the cost of long-distance telephone calls has come down dramatically since the 1970s.
See also: black box