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Securing Voice over IP (VoIP) is a crucial requirement for its successful adoption. Since SIP is the predominant protocol used for signaling to create, manage, monitor and tear down the VOIP sessions, this paper will focus on building a strong defense system around the signaling protocol layer. In this paper, we examine and investigate the concerns and requirements of VoIP security. After a detailed study of security issues and defense mechanisms, we focus on attacks and threats at signaling protocol layer and countermeasures to it that are essential for current and future VoIP implantations. Then, we analyze two popular industry best practices for securing VoIP networks and conclude this paper with further discussion on future research directions. This paper aims to direct future research efforts and to offer helpful guidelines for practitioners.