The Session Initiation Protocol provides an expandable and easy solution to the IP-based telephony environment. When users ask to use an SIP service, they need to be authenticated in order to get service from the server. Therefore, some SIP authentication procedure schemes were proposed to meet the above demand. However, there are security problems that need to be solved, such as off-line password guessing attacks and server spoofing. In this article, we shall propose a new scheme for a secure authentication procedure for the Session Initiation Protocol to enhance the security of the original scheme.