Set up an ''Answering Service'' like http://www.answer-4u.com

Is there an Open Source system you would recommend and would be able to set up for me in order to be able to start to offer an answering service in english plus other languages similar to [login to view URL] ? Ie, I would need all of: the "selling" website with signup possibilities for users that will make calls to the users work directly (if I have a pre-registered number and their redirected number is shown, or if it's not shown, such that the user can choose one of a couple of existing numbers), the actual phone answering system to dispatch calls to agents, and the popup that the agents will see with the info that the user has input so the agent know what to answer, and finally such that the user will be able to pay e.g. 50?'? to a "pre-paid account bucket" from which a price such as 1?'? will be deducted by each call and any additional calling-minutes. The caller should also be able to leave a message with the agent that will be sent as an SMS to the agent for e.g. 0.12?'?