Areski is right,
i had the same problem too but i hadn't upgraded the database after installing 1.4.4.1
here is you have to do:
svn co --username guest --password guest
http://svn.a2billing.net/svn/asterisk2billing/trunk/mv trunk a2billing_1.4.4.1
cd a2billing_1.4.4.1
cd DataBase/mysql-5.x
mysql -u mysqluser -p citivoice_dbname < UPDATE-a2billing-v1.4.2-to-v1.4.3.sql
mysql -u mysqluser -p citivoice_dbname < UPDATE-a2billing-v1.4.3-to-v1.4.4.sql
mysql -u mysqluser -p citivoice_dbname < UPDATE-a2billing-v1.4.4-to-v1.4.4.1.sql
then copy a2billing_1.4.4.1 to your current a2billing directory. don't forget the write permissions acording to your apache user to templates_c inside admin/customer/agent directories.
thanks for the update Areski.