This is a different era of ISP. He was not signing up to have fiber installed to his house or whatever (which was not a thing back then), he was signing up to

This is a different era of ISP. He was not signing up to have fiber installed to his house or whatever (which was not a thing back then), he was signing up to be able to call out to an ISP's server with his modem and get on the Internet from there. So the only thing the ISP knew about him was a username, password and fake billing details, and since the ISP was presumably not a telco, they could also not tell where the call was coming from.