You will need to fully install and setup the latest version of A2Billing on our server.
The choice of OS is upto the devloper. We prefer Centos, but Ubuntu or Debian is ok.
You will also need to setup a SIP trunk we have with our provider and show us how we can add our own providers at later dates.
We will also need to make sure inbound calls work as well with a few DIDs we have.