I have done several projects like this. Your described code g723.1 is not supported by server VoIP Dialer company, be sure before process this project. Although this project will support wide range of codec such as ulaw, alaw, gsm, g729,speex, siren14, g722, siren7, ipc10, g723, slin, g726, ilbc and many more.
Server configuration depend on call processing load. Normally for 200 concurrent call it is recommended to a octa core processor powered dedicated server with 16 GB RAM. We can also help you to taking a server or we can offer you proper server depend on load.
If you want I can show you a running server with this VPN or bandwidth optimization server. Feel free to contact me on skype [login to view URL] or yahoo [login to view URL]