it is a winmodem, but there's a driver released by ess for the chipset you have.
http://www.wiu.edu/users/muslu/essfaq.html
good news is that the modem CAN work with linux, but only with the 2.2.12-20 red hat kernel version. ESS is one of the less linux-friendly companies when it comes to modem driver support. (luckily, they're much better about it in the sound-card area)
if you already have a 2.4 kernel on your computer - it might not be worth it to change over to the 2.2 kernel. you'd probably be better off getting a better modem. if you have anymore questions,
www.linmodems.org is the best online resource for winmodem questions. if they don't have the answer, it's not out there.