I too have an SiS modem, and I found that the instructions on this page worked, after a bit of modification. It's NOT the original installation script, but one I modified to work with gcc3.x, and gave to the bloke who runs this site. Also, I found that it only worked with kppp, as root to start with, but there's plenty on the net about that.
http://radel.inet.net.nz/sis730amr.html
Let me know if it works, or doesn't
G