You could try www.heby.de/ltmodem
where you might find the driver you need or possibly a link to a driver.
You could try downloading Puppy Linux 2.1 or 2.11 to see if your modem is supported by Linux as it comes with SL and LT modem support already configured. (It's a 73MB download which would take about 5 hrs on dial-up) At least you will know if your modem works or not.
I believe Xandros 3.0 free edition comes with the SL modem driver already installed and configured as well.