Hello,
I propose system will be installed as following:
1. OS: Centos 6
2. Asterisk
3. Frepbx
4. Fail2ban
5. A2billing
All will be setup so that you can make call via a2b. I can build a demo in case you need to check it first.
Please let me know if you have any question.
Regards,
Vu Le