In these modern times, dial-up networking might seem a bit quaint. But it still has its place. Many parts of the world still have no access to affordable broadband. A dial-up server is an inexpensive way to provide remote administration access, and to set up a quick and cheap WAN. You may also share a dial-up Internet account; even though that sounds like a recipe for frustration, there are times when it works out. For example, two or three people who don't do a lot of heavy-duty Interneting could get by all right.
You should use a good-quality hardware-controller modem, ideally from a vendor that supports Linux. Messing with cheapie modem drivers on a server isn't worth the pain.