[SR-Users] Prepaid billing solution for a service provider

Hi all, I'm working in a project for a service provider in which prepaid is an essential requirement. They have about 12000 subscribers. The core infrastructure will be Kamailio+RTPProxy while I have some doubts about the prepaid feature. I'm thinking about a B2BUA (SEMS or Freeswitch) that is called if the user belong to a "prepaid group" and perform authorization and accounting; in this case the SIP flow could be: Kamailio -> B2BUA(Prepaid) -> Kamailio. Is a good choice? At the moment I cannot estimate the effort needed to develop this section (base prepaid feature) of the project... Do you have some hints? What are the prepaid solution implemented by you? Do you use your own developed solution or do you think that is better to choose a commercial solution (that provide also calling-card and other features)? I have seen some commercial products (Portabilling, Jerasoft/Bilberry, ...); what do you think about? Best Regards,