is it a usb modem???
post the output of:
it might help to update your slackware kernel (or better yet, the whole install)...
to do that, just download the updated kernel and kernel modules packages (or better yet, the whole directory) to a directory somewhere on your box and then do this (as root) inside it:
remember to run "lilo" (if that's the bootloader you use) after upgrading the kernel...