And here is the physical basis for the LINE you are ON when you are metaphorically ONLINE.
OK well now I am annoyed with Wikipedia. It is most definitely not a "digital" line, it's quite analog, ASR-33's are 100 volts, 20 or 60 milliamps, inductive loads, and generate HV spikes that HURT YOU, lol.
https://en.wikipedia.org/wiki/Digital_current_loop_interface
At least works a bit like this for Minitels as they have a slightly-nonstandard serial I/O, and a phone line plug that they could self-dial.