This little venture can be simple or can be a nightmare, I suggest you visit linmodems at this site
http://www.linmodems.org/ and read up, but better yet hit the colored link on this page called archives to see discussions on many instances of modem troubleshooting, I've seen many pctel discussions there, but the best thing this site will tell you is to download scanModem from here
http://linmodems.technion.ac.il/first.html which will scan your system & tell you what ppp related modules need to be unpacked, what modem package you need, where to get it, steps required to install, how to install, how to test & if you still can't get it to work you'll be able to attach one of the reports this scanModem puts out to an e-mail for discussions and people will help you ( experts ).
PS: the reports this scanModem puts out after running it will be in a folder called Modem in the directory that the scanModem utility resides in.