The thing with phones is so much is built with backwards compatibility or at least similar design principles that any question like this you have to start at the first automated phone switchboards powered by strowger switches.

A rotary phone would issue a number of pulses as the dial spun. 1 pulse for 1, 3 pulses for 3, etc. Each pulse would trigger a strowger switch at local exchange, where it would start turning a dial on the switch with an equal number of turns to the pulses or receives. For a single digit dial it would just have one switch that rotates with the number of pulses of receives. For a much more common 3-4 digit number being entered it would take the rapid succession of pulses to turn the first dial, then after a sufficient pause any subsequent pulses turn the next dial, and so on. Once it runs out of dials to turn on the switch it connects the call to the line which may go to another phone or may simply go to another strowger switch awaiting additional pulses from the user dialing additional numbers.

For example user dials 5-5-5-1-2-3 with a pause between each digit as they dial. The first 5 sends 5 pulses in quick succession to the switch the phone is directly connected to (the local exchange) and that sets the first dial on the switch to 5, the user naturally pauses for a split second creating the pause the switch interprets as a completed dial then the user enters another 5 causing another 5 pulses to go to the switch at the local exchange. After the third 5 it connects to the next exchange likely without the user even knowing and the 1 is transmitted via the local exchange to the 555 exchange where a stroger switch turns to the 1 position from the single pulse, and so on. All of the switches that connect for such a call remain engaged and connected until they receive the disconnect pulse and then they kill the connection ending the call.

With the transition to DTMF tones, much of this same behavior of each switch is just waiting for exactly how many digits it expects then connects the call to the next place remained, and with modern digital and VoIP calls, they continue to emulate the same functionality of the strowger switches, where the exchange is expecting a specific number of digits to be entered, and the user will either enter the correct number of digits or receive an error and a call that doesn’t connect. There are actually still places operating analog telephone exchanges so everything is ultimately backwards compatible, and the security and design challenges of sending signaling over the same wire as voice have remained all thanks to the cost savings choices made by some dude with an amazing mustache in the 1880s. Or we can go even further back because the telephone was actually an innovation on the telegraph, originally designed as a solution for sending multiple telegraphs over a single telegraph trunk line.

Alexander Graham Bell, a pioneering audiologist who worked with deaf kids excitedly penned a letter to an individual at the Western Union Telegraph Company describing using different tones of beeps over the line to differentiate between different telegraphs, and then excitedly went on to describe how with enough different tones one could not only transmit a nearly infinite number of different telegraphs at once but one could theoretically transmit human speech! I’ve read scans of these original letters and you can just see the excitement building as Bell described that part

I’m not trying to be negative about blaming people who are in bad financial situations but idk why more people don’t realize that you can get things that you wouldn’t normally be able to afford if you’re willing to learn about them and do some work.

I’ve been in the spot before and honestly it comes down to risk management. Usually it’s a case of considering fixing something myself, and as I analyze it I end up determining the risk of either being unable to fix it after investing in tools/parts or worse making it worse due to my lack of skill ends up outweighing the cost savings of just paying a professional.

Or for a real world example, I had the rubber gaskets wear out on one of my toilets. I took it apart, dremmeled off the corroded bolts (after buying a big honking screw driver and bolt cutters hoping either would help, and ultimately the bolts were too corroded to unscrew and the bolt cutters couldn’t fit the space to reach the bolts to cug) replaced the gaskets and suddenly have leaks in new spots. Other parts looked corroded so I basically bought completely new innards for the toilet, replaced them, reassembled and it still leaked in the same places. Finally having spent about $300 in parts and tools plus multiple Saturdays of my time, I accepted there must be some art to this plumbing thing that I’m missing and I hired a plumber who fixed it in 30 minutes for about a hundred bucks.

How does the exchange know I’ve dialed the last digit?

This is the fun part: they don’t! The exchange just listens for enough tones to make up an instruction then performs what it was instructed to do. That’s why when you call places it’ll say “press 1 to speak with so-and-so” is you’ve now been connection to another exchange which is waiting for instructions on the form of dial tones generated by button presses.

Phreakers figured out ways to generate the tones needed to all sorts of fun things like play the “payment received” tone into a payphone, or to tell the exchange to connect to another exchange that it might not otherwise (and sometimes would chain them together and see how many hops they could achieve before the sheer distance of the call completely destroyed the call) and all sorts of other fun

Yup it’s also why hold music sounds terrible is the sample rate and ranges are so small there’s basically no music which would sound decent over the connection

What I really loved about Foundation was the sheer timescale of it. Too much Science Fiction is only on a scale of an individual doing something, and maybe it will follow a few individuals over the course of a few decades or even a couple of centuries and you’re left to fill in the blanks, meanwhile Foundation is on a timescale of tens of thousands of years