Next generation IP telephony such as the IP Multimedia Subsystem (IMS) framework has been used to create Internet calling services which let cellular users make and receive calls even when without cellular reception. In this paper, we look at the security aspects of Internet calling services and other systems that use the 3GPP Authentication and Key Agreement (AKA) protocol for authentication, particularly focusing on the context of cellular authentication in Android. We describe a new man-in-the-middle attack on T-Mobile’s Wi-Fi Calling service, which is installed on millions of T-Mobile Android smartphones. We also describe three new attacks on AKA in the context of Internet calling and Android. We have worked with T-Mobile to fix the man-in-the middle vulnerability, and we present clear and actionable solutions to fix the remaining vulnerabilities.