try searching here to see if your modem is supported:
http://www.pcidatabase.com/

And here is the How To for Dial Up modems from Ubuntu:
https://help.ubuntu.com/community/DialupModemHowto

The strong likelihood is that whether it is supported or not - you
could well have difficulties configuring it as i did. To be honest the
easiest and cheapest route is to get a serial modem, install
"setserial" from repos and that's it! I've got three pci winmodems and
"it went against the grain" to go and get another but shop around - a
serial modem costs no more than a pci. I bought two just so i could be
sure of no grief. I use mine for faxing and a serial modem has the
added benefit of just quickly connecting onto any of one's machines.

I must say though i reckon a software modem is more reliable although
slower. One winmodem i have has never let me down with any faxing in 6
years and never a problem receiving.

After all this and experimenting with media and finding my scanner
wouldn't work with linux - i went down the dual-boot route with two
HDs [alas something i didn't want to do!]. So for the sake of a couple
of winblows apps, faxing and a scanner i decided to use dual-boot. A
couple more years and perhaps there'll be more linux drivers for ALL
hardware.

i don't know if this will help you when i first setup Ubuntu on
my machine i had everything hooked up once the operating system was
installed and booted for the first time the operating system
recognized the modem and all hardware hooked up to the computer and
installed the drivers for each if you didn't have everything hooked up
before hand you might want to try installing again but this time hook
everything up first.i use a cable modem and it was there ready to use
after the install.

Same conclusion I unfortunately came to with Gutsy, last Fall, with
essentially the same Modem and Scanner problems. I see no
improvement with Hardy, nor any interest in future developments.

"I see no improvement with Hardy, nor any interest in future developments."
I wouldn't say that quite. As linux use becomes more widespread so
more manufacturers will support it. e.g. Xerox have started to support
printers for linux... yet none of their scanners so far. Which i was
annoyed about as i've just picked up a second hand machine that came
with a Xerox scanner.

If one looks at how linux has developed in just the last two years...
another couple of years and i think we'll see a very different
picture. I use ubuntu as i admire Mark Shuttleworth for what he has
done for the linux community and i think he'll achieve his aim.