I actually have my linux router/fileserver/dns/mail/dhcp server do dialup on a onboard PCtel winmodem. I've also, successfully got that sucker to work ;-) It's not easy, but here's the process you go through..
1: Download the PCtel source/binary drivers for Kernel 2.4. I'd go with PCtel 0.8.6 driver. It's a bit more stable if you dont have an AMR slot modem.
2: Downlaod the newest _stable_ kernel (branch 2.4.XX) and decompress it to /usr/src/linux
3: Configure, and make the kernel and modules (tough if you've not done it before)...
4: Install so a boot option is made for the new kernel.
5: Boot into new kernel (if it works ;-)
6: Decompress the PCtel driver to whereever your home is.
7: ./configure --with-hal=hal && make && make install
You'll need to add a configure option to compile for your chipset.
--with-hal=hal Select one hal in the following list: pct789, cm8738, i810intel, i810sis, via686a.
Yoru modem should be working ;-) Please let Tek-Tips members know if their posts were helpful.