Back in the day radio stations used to have listening lines, telephone numbers you could call to hear what was being broadcast by the radio station. They were used mostly by advertising agencies to check that radio stations were broadcasting the ads that their clients had purchased.That was neat: that problem is still around these days, though you're more likely to confirm your ad's playing over the internet than over the phone.
Joybubbles (then known as Joe Engressia) had a presidential speech for a phone phreak club that tech writers or other educational folks can take as a motto: "knowledge shared is knowledge expanded."
You learn a bit about the frustration of telephone company security folks: the uncomfortable switch between cracking down on hardened criminals and investigating folks who are breaking into your systems out of curiosity. Nowadays, most telephone companies and ISPs seem over-eager to cooperate with government spying; you get to see the history of that. There weren't any laws preventing folks from getting past phone system security and thus avoid being billed for calls. While investigating such, the phone company would listen in on calls and figure out that there were listening to, say, a bookie. So they could get the bookie to stop making calls by siccing the police on them, but had to hide the fact that they were eavesdropping on calls.