ELI5, how do telephone numbers get you to the right person?
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In the old days, you told a switchboard operator which number you wanted, and they physically picked up a plug and plugged it into a numbered jack leading to the right place (or to another switchboard, when calling long distance).
Then (this is a true story), a switchboard operator, wife of an undertaker, began redirecting the calls for another undertaker to her husband. The other undertaker got so annoyed that he invented an automatic switchboard that put the operators out of business. Now you had a rotating dial on your phone, which switched on and off a circuit rapidly and made a connector move a certain number of steps for each number you dialed. Many such connectors placed after one another meant that you could dial 13376969 and get to connector 1 on the first switch, 3 on the second one, 3 on the fourth one, and so on, picking between thousands and thousands of outgoing lines.
Then electronic circuits were invented that could do the same thing without rattling, comsuming lots of power, and wearing out all the time. Now you didn’t need all those pulses from the dial, you could just send a couple of tones to the electronic circuit and it’d instantly know which button you had pushed, so that’s when button phones became available.
Now computers can do it all, and thousands of calls are split into separate packets (very short digital messages, made from ones and zeroes/turning a voltage or light on or off) and sent through a single wire or optical fiber. Each packet has an address saying where it’s supposed to go, so other computers read it and send it off in the right direction. Small snippets of the sound you’re speaking come along in each packet, but all the computers involved, including both of your phones, are so incredibly fast that with just a tiny delay, they can reassemble all of the snippets into a continuous voice conversation.
The place you call from already has a regional code that by default means the next numbers are for the same (roughly ) [continent](https://en.wikipedia.org/wiki/List_of_country_calling_codes?wprov=sfla1).
Then depending on the format, it can cover the population in it. For exemple, North America and and Caribbean ( region +1) has up to 1000 area codes(each covering countries, local areas or a metropolitan area) inside each area codes is a 7 digit number so up to 10 million numbers.
There can be +9 999 999 9999 phone number meaning up to 10 billion numbers.
If the question is about phone exchanges, when you dial a phone number there are either tones or pulses that the computer of your phone company then decodes and sends to the company of the one you are dialing. Over there, it is sent to the right phone company that makes your phone ring.
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