I have two iPhones a 4 and a 5s on my plan. For some reason whenever we back up the 4s to the laptop it uses all the data allowed for the month. It doesn't use the free wifi at home? Can someone shed some light on this?
Yet another possibility and I think you're referring to the 5s, is that Wi-Fi Assist may be enabled. This feature is enabled by default from the factory. To disable Wi-Fi Assist. Go to Settings > Cellular. Then scroll down and tap the slider for Wi-Fi Assist.