The growing use of Wi-Fi®, the proliferation of mobile tablet and smartphone clients, and the advent of solutions to prioritize audio and video using the appropriate QoS have pushed Wi-Fi calling into the spotlight. Wi-Fi calling, also known as Voice over Wi-Fi (VoWi-Fi), has been adopted by platform, infrastructure and device vendors as well as service providers to expand its use into new applications and new services. This paper reviews the evolution of Wi-Fi calling, from the initial implementation as a best-efforts application, to unlicensed mobile access (UMA) and, more recently to a technology based on the IP Multimedia Subsystem (IMS) that can be integrated with LTE. It also discusses how Wi-Fi calling is used in the home, in the enterprise, in public hotspots, and while roaming. As voice services have tighter requirements than data services, the paper discusses how Wi-Fi networks can support voice in an IP environment in residential, enterprise, and service provider networks, and how Wi-Fi Alliance® certification programs ensure a consistently good Wi-Fi calling experience.