I’m trying to understand how an ISP could handle multiple concurrent calls on a single phone number? And did they need a physical modem for each connection? How did this scale to medium to large cities with millions of people? How did the call get transferred from copper wiring between the modem at home, to a modem in an exchange somewhere? What was that modem connected to? Did it vary between countries?
My naive understanding is that there is a direct connection (circuit) between the two modems, but I don’t understand how you could support two concurrent calls on one number.
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How does a corporation or call center handle multiple concurrent calls with a single phone number?
The answer is they use a megalink, which is a digital phone line that can handle multiple simultaneous calls.
A standard one is called a T1 (1.5mb/s) and can support 24 simultaneous calls.
The number call is sent down the link digitally and includes a header that defines the source and destination phone numbers.
The hardware on the company side is a phone system or router that is designed to work with a T1 link as an input and has logic to identify and negotiate incoming calls on different channels on the T1 line, as figures out where to send those calls internally.
Today T1s are falling out of favor in exchange for pure digital links like SIP trunks. This work over the internet but while the protocols are very different they serve the same purpose.
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