In recent years, Voice over Internet Protocol (VoIP) has gained more and more popularity as an application of the Internet technology. For various IP applications including VoIP, the topic of Session Initiation Protocol (SIP) has attracted major concern from researchers. SIP is an advanced signaling protocol operating on Internet Telephony. SIP uses digest authentication protocols such as Simple Mail Transport Protocol (SMTP) and Hyper Text Transport Protocol (HTTP). When a user seeks SIP services, authentication plays an important role in providing secure access to the server only to the authorized access seekers. Being an insecure-channel-based protocol, a SIP authentication protocol is susceptible to adversarial threats. Therefore, security is a big concern in SIP authentication mechanisms. This paper reveals the security vulnerabilities of two recently proposed SIP authentication schemes for VoIP, Irshad et al.' s scheme [Multimed. Tools. Appl. doi: 10.1007/s11042-0131807- z] and Arshad and Nikooghadam's scheme [Multimed. Tools. Appl. DOI 10.1007/ s11042-014-2282-x], the later scheme is based on the former scheme. Irshad et al.' s scheme suffers from password guessing, user impersonation and server spoofing attacks. Arshad and Nikooghadam's scheme can be threatened with server spoofing and stolen verifier attack. None of these two schemes achieve mutual authentication. It also fails to follow the single round-trip authentication design of Irshad et al.' s scheme. To overcome these weaknesses, we propose a provable secure single round-trip SIP authentication scheme for VoIP using smart card. We formally prove the security of the scheme in random oracle and demonstrate through discussion its resistance to various attacks. The comparative analysis shows that the proposed SIP authentication scheme offers superior performance with a little extra computational cost.