Modems to link computers
It is possible to due this between two computers using a phone line simulator, no outside line required. I used to do this when I had trouble with a null modem connection. A company called Skutch makes simulators. The main question is, do you want to connect three computers together to one phone line. This is possible. When my work was poor, we only had a dial-up line for the whole company that the server connected to. The other computers were tied in via ethernet. I hope this helps.