Session Initiation Protocol (SIP) is used for Voice over IP (VoIP) signaling and call control. Billing mechanisms for messaging and VoIP are typically deployed for postpaid services. Prepaid mechanisms for SIP-based VoIP calls toward Public Switched Telephone Network (PSTN) are seldom discussed. In this paper, we propose a prepaid mechanism that can simultaneously process messaging and VoIP calls. This prepaid mechanism is implemented in a VoIP platform developed under National Science & Technology Program for Telecommunications (NTP). Unlike previous VoIP prepaid approaches which require modifications to endpoints, our mechanism can be easily integrated to a VoIP platform without modifying existing network components.