A glitch in Apple's FaceTime app let users hear the other person - and in some cases, see video - even if the recipient never accepted the call.
Anyone can call you on FaceTime and hear audio or see video from your phone before you answer.
The caller can then eavesdrop on the person being called, and in some demonstrations even watch them through the camera app.
Until Apple issues a fix for this, the safest bet is to disable FaceTime on your iPhone this week.
Apple's disabling of Group FaceTime on Monday is a temporary fix ahead of the software update.
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Apple has announced the Group FaceTime feature in the summer of 2018 but then revoked it after early test version of its iOS 12.
The bug emerged on Data Privacy Day when Apple Chief Executive Officer Tim Cook called for "action and reform for vital privacy protections".
FaceTime software enables voice or video calls using iPhones, iPads, iPod Touch, and Macintosh computers.
Apple has also recently been highlighting its credentials as a company that protects user privacy, using a large billboard overlooking the CES technology trade show in Las Vegas last month to declare "what happens on your iPhone, stays on your iPhone".
He commented: "The FaceTime bug is an egregious breach of privacy that puts New Yorkers at risk".
The Electronic Frontier Foundation's Director of Cybersecurity stated: "The FaceTime bug works in both iOS and MacOC".
Users can disable FaceTime by going into Settings, navigating to the FaceTime section, and toggling off the green button at the top of the screen.