Hello, I'm a voip specialist with over 10 years of experience working for a US voip company on the TS level2/NOC departments; I can install, troubleshoot and customize many SIP devices including: softphones, ATAs, IP-phones and PBX systems like Asterisk/FPBX. I read your project description and I can help you troubleshoot and fix your inbound issue, we'll need to setup some tests and capture some packets to find out what is going on. Feel free to contact me at any time and we'll get this project started.
Thank you.