Hello, I'm a voip specialist with over 10 years of experience working on the TS level2/NOC support for a US voip company; I can install, troubleshoot and customize many SIP application including: softphones, ATAs, IP-phones and PBX systems. I read your project description, and I have good network knowledge and may be able to help you with your network setup, you may contact me at any time so we discuss about your network topology idea further.
Thank you.