Hello, I'm a voip tech with 10 years of experience working on the level2/NOC department for a US company; I'm familiar with installing, troubleshooting and customizing many different SIP applications including softphones, ATAs, ip-phones and PBX systems like Asterisk/FPBX. I read your requirements, and I can help you install Trixbox, however, Trixbox is a very outdated GUI that reached EOL in 2012 , my suggestion for you is to install the latest Asterisk with FPBX interface on your VPS. Feel free to contact me at any time to get this project started.
Thank you.