Update: Apple released iOS 12.1.4 on February 7, 2019 to fix the FaceTime issue. FaceTime is now available at the Apple System Status page.
USA Today and the Washington Post have reported that some iPhone FaceTime users have been able to listen to people they are calling before the call is answered. FaceTime runs on iPhones and iPads running iOS 12.1 as well as Macs running macOS Mojave.
Apple is currently working on a fix for this security issue. FaceTime has been temporarily taken down as noted on the Apple System Status page.