system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: how does simultaneous incoming calls work

A house phone can only receive a call in or send a call out. If line is busy then other people can not use it.

When you call different businesses, whether a vanity number or not they are able to receive in multiple calls at one time and line don't normally go busy. Having thousands of people call in simultaneously how does that work but house phones only accept a call at a time?
gpt
The only reason house phones only receive a single call in at a time is because you're only paying for that service. Most phone companies can and will upgrade your home's number of lines if you want them to, or sell you an appliance or service that allows you to handle multiple calls at once by placing one on hold. But most people don't need that, and don't want to pay the extra cost for it, so a lot of people aren't aware it's an option.

The technology is easily there to allow you to have a multi-line phone that places your call in a queue until someone can get to it. Then it transfers you to an individual other phone line at someone's desk as soon as it detects that the person at that desk has hung up from their previous call.  This is how call centres handle their volumes.