Apple iPhone users may be finding an unpleasant surprise in their next phone bill.

FOX News Radio's Brett Larson explains:

FOX on Tech.

Unless you have an unlimited data plan, you may want to ensure a certain setting in iOS 9 is turned off... Called 'Wi-Fi Assist', the feature switches your phones data connection from Wi-Fi to cellular when your Wi-Fi signal gets weak.

Great when you wander outside your home, for example and can't get a solid signal... But if it switches to your cellular data without your knowing, you might be burning through your data faster than you know.

Many users have complained the new feature has resulted in an increase in data use.

You can shut it off at the bottom of the page on cellular settings on the iPhone 5 or newer.

Brett Larson, FOX News Radio.