Install Asterisk

Get the current Asterisk and Zaptel Drivers source:
# cd /usr/src
# wget http://downloads.digium.com/pub/asterisk/asterisk-1.4-current.tar.gz
# wget http://downloads.digium.com/pub/zaptel/zaptel-1.4-current.tar.gz

Unpack the archives:
# tar xvzf asterisk-1.4-current.tar.gz && tar xvzf zaptel-1.4-current.tar.gz

Build the Zaptel drivers (change the directory name to reflect the current version):
# cd /usr/src/zaptel-1.4.4
# ./configure && make && make install

Load the ztdummy kernel module:
# modprobe ztdummy

Build the Asterisk (change the directory name to reflect the current version):
# cd /usr/src/asterisk-1.4.9
# ./configure && make && make install

Install sample configuration files:
# make samples

Make Asterisk start up automatically at boot time:
# make config
# echo "ztdummy" >> /etc/modules

References:
Installing Asterisk 1.4.x on Debian Linux 4.0 (Etch)
http://www.the-asterisk-book.com/unstable/installation-1.4-debian-4.0.html