There are a number of drivers in http://www.medres.ch/~jstifter/linux/pctel.html for modems with PCTel chip sets. To know what driver to download, you will have to know what kernel version you are running. To do that you will have to issue the command
bash$ uname -r
and that will give the version of your kernel.
If you have a 2.2.x kernel, you will have to use the pctel-2.2 drivers . For kernels between 2.4.0 and 2.4.6, you can use the pctel-2.4 driver and for kernel 2.4.7 and above, you can use the pctel-2.4.7 drivers.
But I personally recommend that you use the pctel-0.8.6 driver (the latest version) which is a continuation of the drivers from PCTel with added configure support and some bug fixes.
You may find some other drivers for PCTel based modems on the internet. See section 9.3 for more information regarding these drivers
NOTE: You will find that Jan's site also contain a number of other old drivers. I recommend that you do not use them as they are very much outdated.