linux don't have drivers, I mean, the hardware must be suported by kernel, so you have to llok for a kernel version that supports your modem.
but i can give you a few tips,
1.- is it a winmodem?
if it's a winmodem, forget it, it's not suported, and maybe never will be supported, linux programers don't like this realy bad hardware.
2.- is it a PCI modem?
till kernel version 2.2.19 this kind of modems were nearly unsuported, but i nearly sure that new kernel versions (2.4 or upper) can support this modem
I hope this help you