The best bet, if you have the money, is to buy an older home router/firewall. The older versions have WAN ports as well as Serial Ports, for an external modem. They also have 4 RJ-45 jacks.
Setup is easy, Cat5 into NIC (Network Interface Card) on the computer end, and into an RJ-45 jack on the router/firewall end. External modem into phone jack and the Serial Port on the router/firewall. Power adaptor into wall and router/firewall (that one's obvious, but I figure why not
). Hardware setup complete, moving onto software...
Go to any computer, boot it up. Go to IE, and type in "http://192.168.0.1", minus the quotes. Username + password should be either blank or "admin" (check the literature, it'll tell you). There will be a GUI that should be easy to navigate and add in the correct dialup information. Save all the settings, follow on screen instructions. Software done
.
Once that's completed, depending on how you set it up, you'll be able to go online either by trying to connect to the internet (open IE, log onto AIM, etc), or possibly by logging back into the router/firewall and forcing it to go online (if you want 100% control of when it goes online, if you use that line for calls as well).
That's the proper solution to your problem, and I've had it work in my house for a while no problem, but it's probably the most expensive, and you need what, may now be, obsolete hardware. I'd look into both ways though, and figure out which one will work best for you.