I know nothing about writing drivers, and while it's probably possible to get a winmodem working under Linux, it really isn't worth it. When I first started using Linux, I had a really cheap system with onboard audio/video and a winmodem. I spent days searching for drivers, and when I actually found them, they didn't even work!
Hardware modems work much better than software modems anyway, and since almost all Linux drivers are provided by programmers in the open-source community, they don't want to waste their time making low-quality hardware Linux-compatible. Search for "linux modem" on eBay. I bought a 3COM/USR PCI modem a year ago for less than $20 and it works much better than any Winmodem. You won't regret it!
http://www.modemsite.com/56k/winmodems.asp
