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In recent years, transmission of packets over Internet has been a real alternative to the traditional Public Switched Telephone Networks (PSTN). The Internet protocol (IP) offers more flexibility in the design and implementation of features and services. The session initiation protocol (SIP) is a commonly adopted signaling protocol for Voice over IP (VoIP) by many telecommunication industries. Since the signaling system of PSTN, signaling system number 7 (SS7) was designed for high reliability, but IP works on the "best effort" basis which motivated us to study the performance characteristics of SIP control plane. In this paper, we studied an M/M/1 performance model of the SIP proxy server, showed its limitations, and designed an alternative M/D/1 performance model that enhances the SIP proxy server performance.