Hello,
I can install easily asterisk (from source code or binary, as you wish) and a2billing (last version) on your virtual server. It is a easy project for me. I had 5+ years of asterisk, FreePBX and a2billing experience. a2billing installation includes installation of callback and singup module too. We will need the SSH access to box, MySQL access(in case there is an admin password) and jpg files.
If you have more questions, feel free to contact.