Implementing B-Leg Billing on a2billing

I'm integrating a2billing to my asterisk platform, this system will require me to bill the recipient or the callee for receiving calls. Below is the detail scenario of my application: 1. UserA and UserB both register and connect to my asterisk platform with their extension numbers respectively. 2. Both of them also have profile on the a2billing platform. 3. UserB setup a DID on the a2billing platform with his GSM mobile number as the destination for the DID. 4. UserA calls the userB DID and the call terminates on UserB GSM mobile line via a SIP trunk to PSTN. 5. UserB is billed for the DID to PSTN call. 6. UserA is not billed for anything. The above is what I want to achieve. I've been able to do the setup, but right now, only UserA is being billed for both A-Leg and B-Leg of the calls. But what I want is for only UserB, the owner of the DID to be billed for any calls to his DID that terminates on the PSTN destinated GSM number.