[Users] Help. in OpenSER in Post-paid and pre-paid

Hello Users, I'm Using The Following software of My Sip productions, 1) OpenSER is a Sip Server,proxy, in front-end 2) Asterisk is PBX server, in Back-end 3) Radius for AAA to OpenSER, 4) Asterisk-java for IVR for Billing Customer care

if I did any mistakes in these mail , forgive me .

My Question is

1) If Caller calls to Callee , Before the call established to callee. The Caller has to know his Balance Account. if the Balance is there, then only the call has to establish,

2) I want the OpenSER total in RealTime application Server like Java application . Call Authorization ( Question no 1 ).

If Asterisk is Front-end , ---> problem is Solved,

How can a b2bua implement with OpenSER,

please when