ELI5 : why do some phone numbers require a 1 or 0 before the area code?

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Especially when I’m calling customers at work, I often hear an intercept message saying something along the lines of, “this local call now requires an area code of a 1 or 0 before the 10 digit number…hang up and try again.” I sometimes get the message even if I’m calling someone with the same area code as my own phone number. Why does this happen?

In:

That is so that you can dial long distance. Theoretically any phone number that doesn’t have your same area code requires a 1 or 0 so that the servers can flag the call as long distance.