[gosub100]

I did something similar to my highschool in the 90s. They had a free student phone in the office. It had long distance blocked on it, but I learned you could circumvent the block using those 1010-321 and other long distance prefixes. Some of them had $5 access fees, billed once, in addition to the per minute rate. I called several of these and prided myself on getting the phone removed from the office for a few months.

[fourteenrhinos]

Can you elaborate on why having the phone removed was itself a source of pride?

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?

[gosub100]

I was a rebellious teen. I'm not proud of it now.

[BizarroLand]

Breaking the rules so bad that the ability to even interact with the thing the rule was made for was taken away?

[miki123211]

The Polish spin on this were unsecured office landlines that used radio for some reason, I don't remember if that was for cordless handsets or just an access technology.

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.