First step is to find out exactly what the modem is. Then find out if the modem works with Linux and if so, what the module is. To find out what the modem is bring up a terminal and do:
look for the modem and post what it says on that line.
You might also want to use this page and run the scanModem utility, though I've never used it:
Here is what I wrote a few years ago back when I used a dialup modem. There may be some help for you there. http://linuxbasics.org/tutorials/dur..._configuration
But first find out what the modem is with lspci. If you post it, maybe I'll be able to give you some tips. I ran Debian long ago, too, though I don't know what Debian has now for dialup after you get the module loaded. What desktop do you have on it?