Hello,
I propose your VOIP system will be installed as following:
1. OS: CentOS 6.7 (lastest version)
2. Asterisk: 11 ( latest) or 13 ( latest )
3. Freepbx: Web management for asterisk. This tool will help you easily create sip extension / sip trunk without much skill about asterisk
4. Fail2ban: protect your system from brute force attack.
Please let me know you idea. I'm looking to hear from you soon.
Regards,
Vu Le