Hello,
I'm propose the PBX system will be setup as following:
1. OS: CentOS 6
2. Asterisk: 11 or 13
3. Freepbx: web management for asterisk
4. Fal2ban: protect server from brute force attack
I can show a demo system if you need to check the system. If you have any question, please contact me for more detail.
Regards,
Vu Le