I'm going to assume you're speaking of the 6s because there's no 7 yet. Make sure you have a Dara connection and if you do, check your restrictions to verify they aren't turned on. General > Restrictions
Welcome to iMore! This is an issue related to iOS 9.3. Apple just released an update to fix the problem, iOS 9.3.1. Go ahead and install it and you should be all set.