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The wide scale deployment of Internet combined with several advancements in hardware and software technologies created opportunities for several Internet based applications such as Voice Over IP (VoIP) that involves the delivery of voice, video and data to the end user. SIP, the Session Initiation Protocol, is a signaling protocol for Internet conferencing, telephony, presence, events notification and instant messaging. The SIP Proxy Server is a call control software package that enables service providers to build scalable, reliable Voice over IP networks today and it provides a full array of call routing capabilities to maximize network performance in both small and large packet voice networks. The SIP Proxy Server can perform a digest authentication of SIP Register, invite requests, and can encrypt SIP requests and responses using Transport Layer Security (TLS). For secure communication, user authentication, confidentiality and integrity of signaling message and SIP session establishment are essential. TLS is used for the secured SIP signaling and Secure Real Time Protocol (SRTP) for secured SIP session establishment. This paper is focused on evaluating the performance impacts of SIP proxy servers when secured (TLS) over non-secured transport protocols (TCP/UDP) are used to transport SIP messages. Several experiments were conducted in a lab environment to evaluate key performance parameters such as Call setup time, Mean number of calls, Memory utilization, CPU utilization and queue size.