I've had this happen twice in as many months. Most recently, I noticed that it happened yesterday. Usually, when I'm at home and on Wi-Fi, my iPhone 6 uses AT&T's Wi-Fi calling. I noticed yesterday that I hadn't seen "AT&T Wi-Fi" in the top-left corner for a few days like I'm used to seeing when at home. I went into Settings --> Phone, and Wi-Fi calling was turned off. I know I didn't turn it off and I doubt my wife would have. I'm running iOS 9.3.1 and AT&T carrier version 24.1. I haven't seen any carrier update notifications over the last few days since Wi-Fi calling was last on and I haven't done any iOS updates either. It just seems strange that Wi-Fi calling would randomly turn itself off. Has anyone else experienced this?