In order to compile zaptel package on your system you need to have the kernel source version matching the kernel version that is already running on your system. That can be acomplished by checking the version with following command:
cat /proc/version

Typically you will have similar output:
Linux version 2.4.28 (root@localhost) (gcc version 3.2 20020903 (Red Hat Linux 8.0 3.2-7)) #1 Tue Jan 28 11:01:02 CST 2003

So you need to have the kernel source of 2.4.28 version in /usr/src.
ls -ld /usr/src/linux*
should give you:
lrwxrwxrwx 1 root root 12 Feb 10 2003 /usr/src/linux -> linux-2.4.28
drwxr-xr-x 17 root root 4096 Jan 27 2003 /usr/src/linux-2.4.18

Make sure that you have the config file for the kernel you're running. It should be typically in /boot directory and be called with the version number of your kernel:
ls /boot/config*
/boot/config-2.4.28

Now get into the kernel source directory and create the kernel config file. In order to do that type
"cd /usr/src/linux; make menuconfig"
Load your current kernel config file and exit saving a new config. Now you need to execute:
"make dep"

to create the modversion.h kernel header file that is needed by zaptel. Try to compile zaptel now.