Based on your description, the goal of this proposal should cover to build a present the architecture to deploy a cloud based Hosted PBX Service Provider. It can be done using the infrastructure and services provided by Cloud Service Providers such as AWS, Azure, IBM, Google, Hostway, etc...
I personally have experience deploying proof of concept projects with similar goals, using AWS and providing end customers free of choice for hosted Elastix, FreePBX, SipXecs/eZuce, 3CX and Brekeke.
The architecture includes a Cloud based SBC in order to provide security, transcoding, Security and Survivability features.
End Customer can have full access their virtualize hosted PBX as well as end user portal.