There's a MUD I used to be into back in the day (it's actually still online, and my characters still exist!), and I got several of my friends into it. Unfortunately, most of us didn't have internet at home and could only play it in the school lab.
This was a fairly rural area, and it was the mid 90s. CompuServe and AOL existed, but local access numbers did not - long distance was still quite expensive, and internet time was strictly monitored and enforced even if you were using your infinite free hours on your 12th AOL screen name.
In about '97, a local ISP set up a dial-up point of presence in our town, so the long distance problems were solved. Whoo! However, some of my friends had parents who weren't on board with this whole "internet fad" and so remained offline.
Little shit that I was, I figured out that if you used a terminal client to dial the ISP's number instead of the dial-up networking client, you'd land in a shell; that typically wasn't supposed to happen. Hmmm.....
Normally that console shouldn't even be accessible via the customer dial-up number, but it gets better. You didn't have to authenticate to access that shell, either; just dial the number, wait about 5 seconds after the handshake completes (it's expecting you to start a PPP session at that point), and bam, you land in a shell. From that shell, you could telnet to anywhere on the internet, including the MUD server.
I'm not exactly sure what piece of telecom equipment acted as our game server for so many years, but I want to say it was the router connected to the modem bank. Whatever it was, it was misconfigured in a very advantageous way.
Once I figured that out, the six friends I'd gotten into that MUD were all set as well as a few more people they brought in. HyperTerminal was no zMUD for sure, but it was better than not playing at all.
Looking back, I kinda feel like an ass now. That modem bank was fed by two T1s, one for the modems, and one for the data backhaul, and could support up to 24 simultaneous connections.
It was very common to get the "all circuits are busy now" error message on most days. Making things worse, almost half of the total capacity was us playing MUDs without even being subscribers to the ISP (well, I was, but still).
That worked up until about 2002 when that small ISP was bought out by the local telco who apparently had their equipment configured correctly.
I have a similar story. Back in the 90's I'd go to the library and one of the terminals available auto dialed and logged into gopher (being that this was MN and gopher was home grown). One of the menu options eventually ended me up at another college that had muds in its telnet link sections. But you couldn't really sit and monopolize a terminal at the library. I did see though the phone number written on the modem. So I dialed it from home with my 2400 baud modem. It would just land me at a > prompt. No regular commands worked. But I had remembered that the gopher server was consultant.micro.umn.edu . So I typed that in at the prompt and voila I was connected. The curses client for gopher loaded up the mother gopher site. I found my way to that college with the mud list and played quite a bit. Later I discovered by accident I could just ^C out of the curses client and be dropped to a limited privilege command shell. but it had telnet. So I could just go straight to the muds.
Later I learned from the U of MN gopher site that I could get an alumni account for a few bucks a month. Well, they offered alumni accounts anyway. I hadn't actually attended. I printed out the paper form and mailed it in, just checking the box that said I was an alumni. apparently they didn't check if I actually was, because from then on I had an alumni account. Which didn't really give me anything new except an email address. And I could "officially" dial in like I had been. but I felt like I should at least be paying something.
That's awesome. Now I'm wondering how many people I MUDded with back in the day were also connecting through equally backdoor methods. lol. The answer is probably "a lot".
I also had a 2400 baud modem in my very first laptop (an old 486, 25 MHz, "if you can fix it, you can keep it" model), and would typically MUD through the method I described in my post. It was faster than connecting "officially" since it didn't have to deal with the overhead of TCP/IP and the PPP encapsulation which was very noticeable on that anemic modem. Ah, good times in simpler days.
I always wanted to "war dial" back in the day, but living out in the sticks, it wasn't really feasible since long-distance charges were crazy expensive. Probably for the best, though.