Yeastar Billing Application License - S20. Billing App can efficiently monitor the calls and keep records of each call received/transferred by S-Series VoIP PBX. Use Billing App to allocate call credit and top up extensions, and conduct call analysis on the basis of statistics. Both prepaid and postpaid are supported. Flexible rate settings and detailed records make it extremely easy for enterprise to monitor charges, spot misuse, and enhance efficiency.