The project is to build a virtual PBX system,both the backend and frontend.

instead of writing all required features I would like you to see www.grasshopper.com

This project will be to create the same system.

Basically I would like clients to register and pay
Once they register they can transfer their exisiting number from us or buy a new number from us.
after that the client can configure extensions as they require by accessing their account on the website.
They can also specify where these extensions should ring.

plus all other features provided by grasshopper.com

You would also need to advise what hardware and other equipment I would require.