A bug in FaceTime allows someone to call and listen to you (and possibly even see you) without you answering the call.
Apple has responded by disabling the Group FaceTime feature, which is related to the bug, but that may not be enough. The only way to ensure nobody is listening to you is to answer every call (so you at least know that they are listening to you) or to disable the app.
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Other posts: https://community.isc2.org/t5/forums/recentpostspage/user-id/1324864413
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