You need to load the Windows XP driver for your dial up modem the same way you would a WiFi driver. I don't know if this is something NDISwrapper can do or not, I know Linuxant can do this, you will need to check to see if your modem is supported by their driver loader or not. If it is then the down side of Linuxant is you have to buy a $20 License from them, but with that you get the License for life, and free support from them for life. Here is a link for their page one their driver loader for modems
http://www.linuxant.com/drivers/?PHPSESSID=b6827fd81daa2d2087d3808695b536b7I'm not the best help because I no longer use Dial up and I use cable which Linux loaded a Linux driver for my Ethernet card no problem. There may be a couple in this guild that may be able to give you more details
Side note: I though Ubuntu was "Linux for humans" and they can't even bother answering your question.
blaugh