Hi,
Look at my configuration:
# uname -a
Linux test 2.6.18-92.1.1.el5.028stab057.2 #1 SMP Mon Jul 21 20:55:45 MSD 2008 i686 i686 i386 GNU/Linux
# pwd
/root/zaptel-1.4.12.1
# ls -l /lib/modules/2.6.18-92.1.1.el5.028stab057.2/build
lrwxrwxrwx 1 root root 60 Aug 12 12:09 /lib/modules/2.6.18-92.1.1.el5.028stab057.2/build -> ../../../usr/src/kernels/2.6.18-92.1.1.el5.028stab057.2-i686
# rpm -qf /usr/src/kernels/2.6.18-92.1.1.el5.028stab057.2-i686/
ovzkernel-devel-2.6.18-92.1.1.el5.028stab057.2
# make
...
Generating input for menuselect ...
make[1]: Entering directory `/root/zaptel-1.4.12.1'
make -C /lib/modules/2.6.18-92.1.1.el5.028stab057.2/build ARCH=i386 SUBDIRS=/root/zaptel-1.4.12.1/kernel HOTPLUG_FIRMWARE=yes KBUILD_OBJ_M="pciradio.o tor2.o torisa.o wcfxo.o wct1xxp.o wctdm.o wcte11xp.o wcusb.o zaptel.o ztd-eth.o ztd-loc.o ztdummy.o ztdynamic.o zttranscode.o wct4xxp/ wctc4xxp/ xpp/ wctdm24xxp/ wcte12xp/" modules
make[2]: Entering directory `/usr/src/kernels/2.6.18-92.1.1.el5.028stab057.2-i686'
gcc -o /root/zaptel-1.4.12.1/kernel/makefw /root/zaptel-1.4.12.1/kernel/makefw.c
/root/zaptel-1.4.12.1/kernel/makefw /root/zaptel-1.4.12.1/kernel/pciradio.rbt radfw > /root/zaptel-1.4.12.1/kernel/radfw.h
Loaded 42096 bytes from file
CC [M] /root/zaptel-1.4.12.1/kernel/pciradio.o
/root/zaptel-1.4.12.1/kernel/makefw /root/zaptel-1.4.12.1/kernel/tormenta2.rbt tor2fw > /root/zaptel-1.4.12.1/kernel/tor2fw.h
Loaded 69900 bytes from file
CC [M] /root/zaptel-1.4.12.1/kernel/tor2.o
CC [M] /root/zaptel-1.4.12.1/kernel/torisa.o
CC [M] /root/zaptel-1.4.12.1/kernel/wcfxo.o
CC [M] /root/zaptel-1.4.12.1/kernel/wct1xxp.o
.....
Either you haven't got /lib/modules/2.6.18-92.1.1.el5.028stab057.2/ directory or you run the kernel which is differ from 2.6.18-92.1.1.el5.028stab057.2 but zaptel Makefile gets kernel version from uname -r output.