VOIP system for multiple clients with billing 2

We are looking to setup a VOIP system for clients. These will be setup into preset packages that we can easily create new clients on. They will range from a single line and number to a multi line package with IVR. We would like a control panel that we can create these package easily and add and subtract features. Also each client or package will also need to have access to a control panel where they can only access their particular features and can change their setting such as voicemail find me/follow-me, simultaneous ring, call forwarding, ect. We would also like some sort of billing software setup as well so it will report minute overages or per minute charges or international call charges. We work with open source software or can use licensed, we are looking for both building something that works well and keeps the project price down. We would like to start and keep a long term relationship as we are a provider of services and plan on this growing to multiple servers as well as might need contact for maintenance or changes. Again we want value but at the same time it has to work well and be somewhat user friendly on the control panel side. on a side not we are looking to make sure EVERY "extention" so phone has its own unique number. So anyone that has this service can be assured if they dial this 5 or 6 didget "extention" that they are server to server VOIP and not on a actual phone line. So the clients no it is both a secure, fee and not on a actual phone line. I know this is not a big feature but we want to make sure that it is easy on our side to implement and that if we grow to multiple servers it works. For example on server 1 if ai client dials somone on server 2 “extention it knows to call the correct phone.