I'm not a very technical person, so most points could use some expert expansion. But basically, the way to connect to the internet was by phone - modem, with up to 56k speeds. For the user end they worked much like phones, we used to have ISP numbers and dial them through modem. How it sounded might give you an idea of how it worked, or at least be a bit interesting. So basically, since connection to the internet meant that a number was already dialed, the line was busy. Therefore, you could get no incoming calls, couldn't call from that line, and often if someone were to pick up the phone it would disconnect the dial-up link to the ISP.