The IP Multimedia Core Network Subsystem (IMS) provides real-time multimedia services for Universal Mobile Telecommunications System (UMTS). Through Recharge Threshold-based Credit Reservation (RTCR) mechanism, prepaid IMS services can be supported by the Online Charging System (OCS) in UMTS. In RTCR, when the remaining amount of prepaid credit is below a threshold, the OCS reminds the user to recharge the prepaid account. It is essential to choose an appropriate recharge threshold to reduce the probability that the in-progress service sessions are forced-terminated. An analytic model is developed to investigate the performance of RTCR for the OCS. Based on our study, the network operator can select the appropriate parameter values for various traffic conditions.