Before compiling the Zaptel drivers on a system running a Linux 2.4
kernel, you should verify that /usr/src/ contains a symbolic link named
linux-2.4 pointing to your kernel source. If the symbolic link doesn’t
exist, you can create it with the following command (assuming you’ve
installed the source in /usr/src/):
# ln –s /usr/src/`uname –r` /usr/src/linux-2.4
Computers running Linux 2.6 kernel–based distributions do not usually
require the use of the symbolic link, as these distributions will
search for the kernel build directory automatically. However, if you’ve
placed the build directory in a nonstandard place (i.e., somewhere
other than /lib/modules/<kernel version>/build/), you will require the
use of the symbolic link.

vzctl mount NNN
mkdir -p /vz/root/NNN/lib/modules/`uname -r`/build
cp -r /lib/modules/`uname -r`/build/ /vz/root/NNN/lib/modules/`uname -r`/build

ln -s /usr/src/linux-2.6.8-1.521 /usr/src/linux-2.6
ln -s /usr/src/linux-2.6.8-1.521 /lib/modules/`uname -r`/build