Session Initiation Protocol (SIP) based Voice over Internet Protocol (VoIP) services has attained much attention over the last decade. SIP is an extensively used Internet protocol for real time communication and establishment of media sessions. However, it is vulnerable to several security attacks due to its open architecture and text-based nature of SIP messages. Also, the inherent vulnerabilities of the underlying transport protocols such as TCP, SCTP and UDP renders SIP exposed to some serious security flaws. One of these is the protocol’s weak authentication scheme that leads to a number of attacks including registration hijacking, impersonating a server, message tampering, session teardown, DoS etc.