As a popular application layer signaling protocol, Session Initiation Protocol (SIP) has been adopted by telecom NGN, 3G IMS, and various Internet and enterprise applications. The payment mechanism is an important enhancement for SIP services and can bring economic incentives to the service providers. This paper proposes SIPCoin, a new real-time payment scheme for SIP services based on hash chain. SIPCoin takes advantage of the efficiency and irreversibility of the hash chain. With a simple extension to the SIP protocol and a new payment message flow designed in a standard way, SIPCoin integrates hash-chain-based micro-payment mechanism with the SIP session establishment and management process to provide real-time payment for SIP services. Compared with present payment schemes for SIP services, SIPCoin not only has higher efficiency, better real-time performance and extensibility, but also enhances the security by preventing electronic currency from forgery and thievery.