Mobile Voice over IP (Mobile VoIP) allows mobile users to continuously talk with each others while roaming across different networks. In such wireless application, it is vulnerable to information security. In this paper, we present two efficient and secure authentication protocols for mobile VoIP services. In our approaches, we first develop a secure authentication protocol for mobile IP registration procedure. And then, based on this authenticated process, we extend the similar concept to a SIP-based VoIP service, and thus propose the second simple and secure communication protocol. For verifying our contributions, we demonstrate our proposed protocols secure and efficient: (1) by conducting security analysis on the aspects of data integrity, confidentiality, and several common network attacks, such as replay, man-in-the-middle; and (2) by conducting simulations and showing comparison results with other existing protocols, in terms of the communication and computation overheads impos...