There is a company called Conexant makes cheap chipsets which are built into a lot of modems with different brand-names.
There is now another company Linuxant which produces Linux drivers for the various modems which are built on Conexant chipsets:
http://www.linuxant.com/drivers/
This website will identify if you have a Conexant chipset modem and supply a free basic driver (limited to 14kbps data - try this one first for compatibility) or a paid-for driver ($14.95 for 56kbps with fax).