Welecome to LQ.
Getting down to businbess.
Are you *sure* you have the kernel source installed?
you really need to install your kernel sources. The file
/usr/include/linux/modversions.h is the wrong file (even though the name is
the same). Usually, kernel sources are installed in /usr/src/linux*.
After installing your kernel sources, you sometimes need to configure
them, before a modversions.h file is generated (depends on your
distribution).
The file you want is not the one in /usr/include/linux, it is in
/usr/src/linux-2.4.x.x/include/linux which is symlinked from
/lib/modules/2.4.2-2/build/include/linux.
Also check out this
http://www.infologics.50megs.com/PCtel%20modem.txt
Cheers