A few weeks ago I needed to start a small call center for my business and I wished to use VoIP technology to reduce phone costs. I needed to consider the billing types service providers use. I tried Ozeki VoIP SIP SDK and it was very useful for my business because I could choose between postpaid and prepaid billing systems. By prepaid billing you have an account that you can fill up by paying for credits that can be used for phone calls. In this case when you call a number your credit decreases according to the call. You can only talk for the time your credit is valid. However I chose another option. By postpaid method I can pay by getting a bill or making a money transfer. There is a given amount of money I pay for the service and above I pay for the calls my call center agents made.
So if you are interested in this billing method as well you can check it google-> voip-sip-sdk.com/p_462-voip-postpaid-billing-voip