Thanks for the suggestion, it looks interesting. If I understand correctly, the IVOZ application is a Multitenant PBX. It's an integrated billing solution that's built around Asterisk 13 and PJSIP but I want to move away from Asterisk to Freeswitch so this won't work for me. My goal is to use fusionpbx with a billing engine or system also based on Freeswitch.
The billing engine part IS Freeswitch. It uses a number of components. Though yes, asterisk will be there its something you need not touch or use in any way. That's just if you want to use the PBX features.
The billing system is actually CGRates/Freeswitch.