Well, I don't know about USA, but here in Canada there's something called Internet Call Waiting, where dial-up users subscribe to it and they can get a pop-up message whenever an incoming call comes in. You can either accept (where I think it will disconnect and you can answer the call), reject or play a recording to them.
I was dying for that kind of thing back in my dial-up days. Ever since I got cable, the phone thing was never an issue.
As far as voice and data being on the same phone line, isn't that what DSL does?