We old, man. I remember as a kid my uncle in Minnesota was on what they called a party line, wherein I guess a bunch of his neighbors and he were on the same line sorta, but each phone had a different ring pattern. So you could easily eavesdrop on your neighbors' conversations if you wanted to.
And speaking of that, early cordless phones, probably in the 40-50MHz range. I had a scanner that could pick those up too. Just kind of stumbled upon that by accident one evening.
I did comm for the Marine Corps so the wire/switched backbone side of that is kind of a time capsule of telephony, depending on what you're doing.
At, like, an infantry platoon level you have the "party line" level of technology--also called a local loop. The ubiquitous green metal phone with a pair of C cell batteries, in a zippered canvas case. Splice your speaker wire onto it and onto another phone, turn the crank to generate a ring. In WWI, before radios, they'd use them for patrols. The patrol would go out with a phone and a spool of wire and when you hit the end of the wire you were at the end of the patrol. Or you can splice them together into a "gun loop" so your command post can talk to your machine gun positions and maybe your forward observation post. Was there a way to turn off the ring? I forget. Maybe you didn't run one to your observation post. But yeah, you use a ring pattern so someone knows who should pick up. There was also a voice-powered phone that didn't have batteries to boost the signal. Barely a step up from 2 tin cans and a wire.
Next step up was the SB...22? I think SB-22 switchboard. This was the equivalent of the old-timey operator. You had a lunchbox sized green thing with a half dozen patch cords and an operator sat there and patched you to whoever you needed to talk to. I mean, as long as they were at one of the other 5 locations wired to the switchboard. Was there a way to tie it into a more advanced switchboard? I forget.
After that was a genuine automatic switching device. I think it was still analog and very basic. It was kind of the "middle kid" when I was in (early 2000s). It was too advanced to be in use at an infantry company and not advanced enough to be in use at a battalion HQ. But I feel like you could actually dial numbers on that instead of needing an operator.
After that was the SB-3865. Cold-War era digital switching. Able to support secure encrypted phones. Programmable. About the size of two rock concert road cases. The fun thing about that was, just above the control panel and display screen (which I remember being about 5" square and green or brown) was a big red button. Since it was developed during the Cold War, the idea was, if your position was being overrun by the Reds, you pushed the button and it melted all the circuits in the switch, making it useless. Now, psychologically, it's a terrible idea to put a big red button right at eye level to a sleep-deprived 20 year old, so the first thing you do when setting it up is duct tape over the button.
Up from that was the...TTC-42(?) "van"--4x the capacity of the 3865 in an actual enclosure with room for 2 people to sit and monitor the switch.
Then there were some newer devices at the general officer command level that I hadn't been trained on that could do everything that these older systems could, but in practice it was useful to have the older systems wired in for redundancy. That way if the new stuff went down you didn't completely lose your comms.
Oh, then there's the wire: We had "slash wire" which was basically just cheap speaker wire. Then we had "26 pair" which was essentially 26 pairs of speaker wire, bundled together in a rubber sheath with contacts that could be connected together to create a longer wire or connected onto a switchboard. There was tactical fiber, which was fairly fragile and unreliable. And then there was the MUX. Multiplexed UHF radio. Looked a little like a green Humvee version of a live TV news van. You put up a mast with a dish on it and pointed it at its counterpart at a different location, got the dishes pointed right at each other at the same height and when you got everything tweaked just right you could send voice or data over air instead of having to run miles and miles of cable. Fun times.
And I can safely talk about all this because it wasn't actually classified back when I was in and at this point it's the equivalent of someone telling you about building Websites by writing HTML code in Notepad on a Windows 95 computer. I'm certain things are much more advanced these days.