Apple have confirmed that they have fixed the FaceTime flaw that was discovered by a 14 year boy.
You must update to the latest version on your supported device (12.1.4) before you switch FaceTime back on again.
The issue was discovered to be in the group chat feature, even if you didn’t use it yourself. Anyone who called you, using Facetime could simply press the button to add another user to the call and then enter their own number and they would have access to your microphone and forward facing camera allowing them to see and hear you, even if you had not accepted the call.
A very worrying security flaw. For those of you who disabled the FaceTime feature on your phone, iPad or IPod Touch, you can now turn it back on again after performing the update.