[SR-Users] no hangup when there is no network or when the UAC on the other side is power off

Hi everybody, We are using kamailio+callcontrol+cdrtool+freeradius+mysql to build a voip prepaid based system: . 2 SIP accounts created and one of them is prepaid . we can debit balance when we call with the SIP prepaid account, and when the balance is 0, the call hang up . if we hangup a call session from the prepaid SIP and the balance is not still 0, the call hangup and the system debit the appropriate cost . but when we call from the prepaid SIP account, and we disconnect the ethernet cable or the alimentation of the other UAC, the call don't hangup until the balance is set to 0 . if the phone is still ringing too, when we disconnect the other UAC, the caller UAC still ringing . the 2 uacs and kamailio are in the same subnet network the question is, how can we tell to kamailio to detect if the other side is down and then send bye immediately so thecalculating of the credit will be correct? Thank you. --