But unknown to many of those users is that one feature, called Wi-Fi Assist, uses data to supplement a spotty Wi-Fi connection -- an unfortunate and ironic twist for people taking advantage of Wi-Fi to save on data usage.
Wi-Fi Assist automatically connects to a cellular data network to boost connectivity whenever a Wi-Fi signal is weak, according to Apple's website. Because it automatically determines when it's necessary and flips on and off on its own, users are unlikely to know when Assist is in effect. And while that might not matter much to people with massive or unlimited data plans, users with limited data plans could tally up a significant amount of overage fees if their phones are constantly using the feature.
How to turn it off? It's easy, but a little buried. Just go to "Settings," then head to "Cellular," scroll down to the bottom of the options, and de-select "Wi-Fi Assist."