I think you could make a business out of developing a module on top of Asterisk that enables wholesale hosted PBX services. Asterisk is not only being used for retail hosted PBX but also wholesale hosted PBX as well. I think the privately brandable hosted PBX services of Vox Communications use Asterisk as the underlying platform. But I am not 100% sure.

Vox must have developed in-house the software that lets them offer smaller service providers a privately branded hosted PBX services. Imagine if they license that piece of software out to other wholesalers as well. That is the creative idea of the day. Sylantro and Broadsoft have made some dollars selling such software. The fact that the developer starts with an open source feature server gives a significant advantage to a newcomer. Such a software is easily extendable – conceptually – across other areas as well. For instance you could do a module (on top of an open source CRM platform) that enables wholesale hosted call center services as well. The difference of course is that the developer is leveraging open source platforms and has good chances of monetizing the solution.