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?
At my last apartment my LL would only allow a single number per apartment... well I was sharing the apartment with someone else and I was sick of being the only person to get called. 30 seconds of Googling revealed the user manual for the intercom, and of course the default password of "5555" was still set on it...
I programmed both our lastnames and phone numbers to our apartment unit number. I did that in 2014 and I moved out in 2016.
To this day -- NINE YEARS AFTER MOVING OUT -- I am still getting calls whenever someone hits #25 on that intercom.
I should have done the 1-900 thing :D
So what they did is add a new fake tenant with a premium 1-900 number and used the intercom to call it, earning themseleves a bit of cash. Naturally, landlords had to foot the bill.