Here are my thoughts on the situation.
Buy a cable/dsl router with dial-up backup (the SMC routers I reviewed on my site have this). Basically, you would connect your computers to the router via ehternet, and then connect a PC modem to the router. Set up the router to use the modem, and it should connect any time you don't have an ethernet connection to the outside world (this would be always). Of course, this setup would probably keep your internet connection active 24 hours a day, and that may add up on your phone bill. You could always shut off the router when you don't want to be on the internet.
While I have never tried it, I would assume it would work.