@cognitiveinequality Interesting concept, I'm going to use this as inspiration to talk about the history of telephone signalling. Sure, you could make up cyberpunk stories, but there is so much interesting history that actually happened, and technologies at work under the hood that have been supplanted by time.
Telephone systems have to have methods of signalling. The two most common you hear of are:
Pulse dialing (like old rotary phones) - Quick contact closures, about 10 pulses per second. If you were good enough at it, you could rapidly actuate the hook switch and get the same effect. Dialing a number 5 results in a switch closing and opening in rapid succession, which in the early days would make a fancy solenoid physically increment a ratcheted mechanism 5 times. That's a gross oversimplification, but you get the idea. This system dates back to 1892, when automatic telephone dialing was in its infancy, and everything was electromechanical.
DTMF (dual-tone multi-frequency, also known as touch tone) - Two tones overlaid on top of one another. Using 8 frequencies, 4 on the rows, 4 on the columns, the combinations correspond to specific digits. However, the 4th column frequency, 1633Hz, is reserved for special usages, and was rarely used. Dialing the number 5 is a mixture of 1336Hz and 770Hz. It wasn't introduced until the 1960s, when electronics had taken over and could detect individual tones to decode the numbers being pressed. The result is a much faster dialing process.
These days, cell phones use out-of-band signalling (meaning you can't hear the signals being sent over the voice channel), and send the entire telephone number at once, rather than supplying a dial tone and making the user enter the the number a digit at a time. But that's boring, so here's more traditional in-band inter-office signalling methods used in the days of yore.
Early Bell System automated telephone exchanges like the Panel switch from the 1910's used what was called Revertive Pulsing to signal other exchanges when doing more complex dialing, both for short and long distance calls. RPing is best thought of as the originating office saying to the next office "okay, you start counting, and I'll stop you when you get to the right number." So the far off office start's moving a motor, sending back a pulse as it passes each terminal along the switch, and when it reaches 5, the originating office signals for that motor to stop where it is, and then it moves on to the next digit. You could sometimes hear the pulses on the line as the switch worked its magic, assuming you lived somewhere with a panel office, or called into one.
Multi-Frequency signalling is a similar concept to DTMF, except it only use 6 tones, but uses them far more efficiently. It was mostly used by long distance operators at first, manually pressing keys to signal not only numbers, but other commands to the switch, like "keypulse" and "start tone". Later, it was common for telephone exchanges to automatically send these tones, not just human operators. If you've ever heard of a Blue Box, this is the type of signalling it's intended to perform. People heard this type of signalling on the line pretty commonly during long distance calls, usually as a faint background sound. A perfect example can be found at the end of Young Lust by Pink Floyd (on the album The Wall), wherein the operator is talking with someone trying to place an overseas call.
Single-Frequency signalling is basically like dial pulsing, except instead of contact closures you're sending bursts of tone over the phone line. Namely 2600Hz, but also 1600Hz, or 3600Hz depending on the system.
2600Hz is an important frequency when it comes to long distance trunks (the special groups of telephone lines between exchanges, mostly those more than a county away). A long burst of 2600 would seize a trunk and let one telephone exchange use it to talk to another exchange on the other side. Once in control of the trunk, the central office would send along MF tones to direct the call on the next leg of the journey. And if you had a way of generating that yourself, you could assert control of the line, and route the call another way (assuming you had a Blue Box). Thus, the number 2600 has maintained a significance long after it served any purpose in telephony. 2600: The Hacker Quarterly is one such magazine paying homage to the history of hacking.
Parallel to this, Modulator-Demodulator's became a thing, more commonly referred to as a modem. Modems using telephone lines is something that goes back to 1958 with the Bell 101 type modem intended to be used by SAGE. It operated at 110baud. Eventually, you could purchase the Bell 103 starting in 1962, which operated at a whopping 300 baud and used frequency shift keying. However, consumers wouldn't start interacting with them in a practical sense until the 1980s.
Oh, and then you have teletypes (like a typewriter but connected to a phone line) using telegraph and telephone lines. The history of making the telegraph faster and finding better encoding methods goes way back into the 1800s, and that's where you get stuff like Baudot using a five bit descendant of morse code, that eventually would be modified for use on something with a keyboard in 1901. Then you ended up with telex and other services sending text to teleprinters to get information across a country in no time, but that wasn't something regular every day people would generally interact with. Closest you would come to that is sending a telegram, which someone else would condense, send over telegraph and later teletype, then put on a piece of paper and deliver. But those were mostly traveling over dedicated lines. Eventually, you saw teletypes being connected to modems and used to talk to early computers over long distances. Something you might see in a university, or at an important business in the 1960s or 1970s.
Okay, that's enough rambling. I think I named all of the major in-band signalling methods. Someone let me know if I forgot something.
This stuff is just... so.... COOL!
