Modem not working under linux has always been very, very common. I haven't used Linux in years, but back in the day you could ONLY use REAL Hardware modems. Which shipped with absolutely NOTHING. (I don't think a single new system ever shipped with a hardware modem once they invented Winmodems). I actually had a few 56K hardware modems (several ISA ones, and an external one), but I didn't know anyone else who owned one. I eventually started chucking the good ISA ones into other people's systems I would work on (people who still USED modems, I haven't dialed up anything in like 7 years).
On a Winmodem rant. Arrrgh, I have an entire BOX of unidentifiable winmodems. Countless times I would have to work on someone's computer that was completely horked, and after the new OS install I would not be able to get the Winmodem setup. There would be like 90,000 different models of them that all had the same numbers on them, only the RIGHT driver would work (and the Manufacturers websites were useless, a given system might have one of 10 or 15 different modem drivers listed, none of which would ever work). It got to the point that when someone wanted a system redone I would tell them to bring me the system and a brand new modem in a box.
Apparently XP is better about that now, the few times I have set up XP on a system that actually had a modem, it caught it right off the bat.