Sounds as if Vista may be sending a different initialization string to the modem than what Linux is. Have a look in the modem properties in Vista and see what init strings may be listed, then use those in your Linux setup.
Another thing you can do is try Google for
modem help. I found a site I used to use when I was on dial up for this sort of thing:
http://www.modemhelp.org/inits/
Another good site is
http://modemhelp.net/
Hope this helps.
