#I managed to resolved the problem for installing callback-daemon on centos, version 1.7.1 of a2billing. (this should work for other back dated version)
#I am assuming you've retrieved the trunk package using svn and stored all the files under /usr/local/src/a2billing/
#Also your a2billing.conf file is stored here --> /etc/a2billing.conf else take note of the additional steps marked in red
#goto target directory
(1) cd /usr/local/src/a2billing/CallBack/callback-daemon-py/dist/
#untar the r1528 file to create callback_daemon-1.0.prod-r1528 directory
(2) tar -xvf callback_daemon-1.0.prod-r1528.tar.gz
#Go down into the newly untared directory
(3) cd callback_daemon-1.0.prod-r1528
#excute the following python commands as per a2billing doc site
(4) python setup.py build
(5) python setup.py bdist_egg
#additional steps to correct the path for the a2billing.conf file
(5b) vi callback_daemon/a2b_callback_daemon.py
(5c) replace the path to which your a2billing.conf resides in your system, the line should be --> CONFIG_FILE =#execute the installation
(6) easy_install dist/callback_daemon-1.0.prod_r1528-py2.4.egg
#install the init script and daemon service (still at your current directory level)
(7) cp build/lib/callback_daemon/a2b-callback-daemon.rc /etc/init.d/a2b-callback-daemon
(8) chmod +x /etc/init.d/a2b-callback-daemon
(9) chkconfig --add a2b-callback-daemon
(10) chkconfig a2b-callback-daemon on
# finally start the service u can either use the init script or service command
(11a) service a2b-callback-daemon start
(11b) /etc/init.d/a2b-callback-daemon start
#hopefully this solves your problem