Have to endourse most of what zeugma has said.
While he cannot speak from experience with dialup on Linux, i can. fuggedaboudit! i speak as a dialup user currently posting from SuSE 9.2 Professional.
First off, you can forget about most PCI (internal) modems. Even the ones that have drivers available for them often require a recompile of the kernel...not something that the average user wants to try.
Second, my experience with SuSE's dialup applications was a nightmare. It seems that those particular packages were not upgraded, and had serious flaws. i finally got kppp to work by copy/pasting a file into a directory (that also did not exist) in the application. i had to use an old external Zoom Telephonics v90 serial modem that did not require a software driver.
i stayed with it, spent more time than i care to think about on Linux boards, and Google, and eventually did get it to work, but let's be honest: It is definately difficult if your hardware is not recognised. Most internal modems are called Software, or winmodems, and are designed to work only with Windows. Until more manufactures start to design Linux capability into their modems, dialup will continue to be a nightmare. Others here have given up on the ideal of Linux dialup, and have come up with some unique solutions (while still maintaining dialup internet service). You may wish to talk to some of them.
http://www.ipcop.org
With an external modem it worked as good as dial-up can be.