Quality in VoIP billing is measured in three areas:
1. Features and Flexibility
2. Scalability
3. Redundancy and high availability (if you’re doing prepaid or real time credit control)
For VoIP billing checkout MindCTI, Alepo, Portal, and Intec. Those are the leaders.
Don’t fall for new companies with nice GUI but open source software under the hood.
When the references include incumbent carriers it is also a good sign.