You can go here
and follow the scanModem tool link, read as you go. Run the scanModem tool and it will produce all kinds of info regarding your modem and where to get source code, what to install, what to do, etc.
If it is a Conexant, you'll only get turtle speed till you fork out the bucks. Dell did put out some models of laptops with Ubuntu installed, the drivers for these models available from Dell's site give full speed.