http://www.linuxquestions.org/questi...ht=pctel+hsp56
... there is this really cool "search" function... this is only one of thousands of similar replies to exactly your question in different distributions.
What I would suggest is running the scanmodem script from the linmodem project and reading through the files it generates. For that matter - reading about linmodems.
You should know that RH8 is wildly depreciated. You should use at least RH9 ... but unless you have a compelling reason to use such an outdated OS, you are strongly urged to consider a more recent distro.
For a start, many of the latest drivers may simply not work with RH8.