Hello, I'm a voip specialist with over 10 years of experience working on the TS department of a US voip company; I can install, troubleshoot and customize many VOIP devices including: softphones, ATAs, IP-phones and PBX systems like Asterisk/FPBX. I read your project description and I can certainly help you setup and configure A2billing. Feel free to contact me at any time through chat for further talk.
Thank you.