[Serusers] ser- b2bua config/scenario question

Hello! Plese help me, if possible! I have a SER server and Vocal b2bua on my linux server. I would like make a simple prepaid application whith these components. I'm beginner in SIP, therefore excuse me for stupid questions. SER works fine without b2bua, sends accounting messages to the freeradius server, but can't hang up conversation depend on user balance amount. Therefore i need a b2ua, that watches the sessions, and starts a timer (vocal 2b2ua be able to do it ) How works b2bua in this plan? Need i one b2bua between two SER servers for calltime controll? Are there any other simple free solutions for prepaid billing with SER? Is there anyone who use b2bua with SER server? If yes, please send a configuration hint for me! Thank you. Regards, Antal Pletli email: a.pletli(a)euromacc.hu