An Apple iOS 9 feature for iPhones as well as iPads with cellular service. Wi-Fi Assist switches data transmission from Wi-Fi to cellular as the user moves out of a hotspot.
Wi-Fi Calling Is Cellular to Wi-Fi
Whereas Wi-Fi Assist switches from Wi-Fi to cellular when necessary, "Wi-Fi calling" uses Wi-Fi instead of cellular to make a voice call (see
Wi-Fi calling).