Freeswitch Multi-tenet PBX with ZRTP and GUI (possible blue.box or FusionPBX) and billing

Freeswitch Multi-tenet PBX with ZRTP and GUI (possible [login to view URL] or FusionPBX) and billing We are a VOIP provider and want to create a new PBX client setup. We are looking for a muli-tennet (multi-organization). We need a easy GUI for both us internally and our clients. We were looking at [login to view URL] or FusionPBX but are open to other ideas as well. Beside the normal PBX features we would like to be able to optionally allow the option to give clients ZRTP license protection. The most common setup we do for a client is a 3 – 5 extension with a main IVR and one conference extension. They should be able to have a call transfer Lastly we need a simple billing or at least something that will give us a per min usage for each client.