Hi ,
y dont u try to use the sm56 modem on any version of linux by running the redhat 7.1 kernel ?? , ie .. install redhat 8 or 9 or whatever distro.. force install the old kernel
rpm -ivh kernel-2.4.2.i686.rpm --force
them edit ur grub to boot into the 2.4.2 kernel
(of course ext 3 filesystem wont work )
but if u still use a ext2 filesystem ur in for some luck i also suggest that redhat should give an option to stick to the present kernel during an upgrade . this can save a lot of headache !!
btw i am not any kernel guru or anything but i feel that 2.4.2 works more faster than 2.4.18... .. my system is more faster on 2.4.2 kernel ..

cheers to all