The problem is believed to affect any pair of iOS devices running iOS 12.1 or later. It allows a someone starting a group FaceTime call to listen to your audio without you ever knowing. Apple say it will be addressed in a software update later this week. If you are concerned you should disable FaceTime in iOS Settings.
For more information read 9to5Mac’s article – Major iPhone FaceTime bug lets you hear the audio of the person you are calling … before they pick up.