People would walk around big cities, usually on Friday evenings, radio scanner out, trying to find one of these. They would then dial a premium-rate number, preferably on more than one line. In most cases nobody would realize that something was up until Monday morning, and if they had a way to disconnect the calls before then, not until the bill came.

You could do similar shenanigans with unsecured PBXs or insecure answering machines that had a "call my mobile if somebody leaves a message" feature.

I do appreciate the hacking around aspect, particularly with respect to old phone systems, but having a free student phone removed seems like it would be a bad thing for everyone, no?