For a single Quintum Tenor buying your own real billing solution would be an overkill. And buying the “complete VoIP billing for $500” is just a waste of money. I would suggest going for a hosted billing solution, where you can get a good VoIP billing for relatively low monthly payment, and using it until you will have enough traffic to pay for your own system. There are plenty of providers who can provide hosted PortaBilling or VoiceMaster.