You need to download the kernel source, and make two folders, linux, and either linux-2.4 or linux-2.6, (depending on which kernel you have, 2.4 or 2.6).  Link these two folders to your kernel source directories.

cd /usr/src
ln -s [kernel-source-dir] linux
ln -s [kernel-source-dir] linux-2.4

OR (DEPENDING ON YOUR KERNEL VERSION)

ln -s [kernel-source-dir] linux-2.6

Make sure that where [kernel-source-dir] is, you use the full directory path for the kernel source.  Next try to compile Zaptel.  If compiling fails, issue the following command, and make sure that where [kernel-source-dir] is, you use the full directory path for your kernel source.

cd /usr/src
ln -s [kernel-source-dir] /lib/modules/`uname -r`/build

If Zaptel still doesn't compile, issue these two commands, and then try to compile zaptel.

ln -s /lib/modules/`uname -r`/build /usr/src/linux
ln -s /lib/modules/`uname -r`/build /usr/src/linux-2.4

OR (DEPENDING ON YOUR KERNEL VERSION)

ln -s /lib/modules/`uname -r`/build /usr/src/linux-2.6

If that does not work, or you don't have a build directory in your /lib/modules... directory try the following:

cd /usr/src/[kernel-version]

make mrproper
cp /boot/config-[kernel-version] .config
make clean
make oldconfig
make dep

If 'make dep' reports that its no longer needed, you'll need to execute the following commands.

make include/asm
make include/linux/version.h
make SUBDIRS=scripts

Before compiling make sure the EXTRAVERSION variable in the kernel-source Makefile matches the running kernel.

vi Makefile
/EXTRAVERSION (this will search for ExtraVersion in the document)
EXTRAVERSION = -(kernel suffix)

Save the file, then exit.  You should be able to compile Zaptel, Libpri, and Asterisk.