Session initiation protocol (SIP) is the most prominent signalling scheme for controlling, establishing, maintaining and terminating communications on the internet. Many researchers are working to create strong protocols for secure communication using SIP. Tu et al. (2014) proposed an improved authentication protocol for session initiation protocol. They show that their scheme can resist a number of security attacks. However, we find that their protocol suffers from inefficient login phase, inefficient password update phase, impersonation attack and unable to keep the user anonymity. To make all these types of attacks ineffective, we present a very efficient scheme for SIP. Security investigation of the proposed scheme shows that our scheme can make all the shortcomings of Tu et al.'s scheme as ineffective.