Apple have announced that iOS 12.1.4 will be released next week. This patch is essential for anybody running iOS 12.1 or later as it will fix a FaceTime bug that enabled other iPhone or iPad users to activate a FaceTime call without the consent of the person receiving the call, thus, giving the ability to listen in on the other iOS user.
Apple have published on their developer website that as of 3rd September 2018 85% of compatible devices are running iOS 11, which is currently the most up-to-date version of their mobile operating system. It is widely expected that iOS 12 will be release later this month. (Apple's Developer Website)