The company is now addressing the issue and has temporarily taken Group FaceTime offline.
The bug was discovered on 28 January 2019 when a user was able to set up a Group FaceTime call that enabled them to see and hear a recipient even when the recipient did not answer the call.
The bug made it possible for the caller to call a contact and another to add themselves as an additional participant to a group FaceTime call. While the call was ringing, the microphone would be switched on so the caller could the intended recipient even if the phone was locked.
The call can be rejected by pressing the power button on the side of the phone but the camera would switch on by doing so. The caller could listen in and watch for as long as the phone is ringing.
Apple is expected to release a software update this week to address the issue. Until then, Group FaceTime will remain unavailable.