Apple has acknowledged a flaw in its FaceTime software that allowed for brief eavesdropping - even if the recipient did not pick up.
In some cases the target iPhone could send video without the receiver's knowledge.
The company said it had developed a fix and an update would be rolled out this week.
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In the meantime, Apple's status page shows it has disabled the ability for users to make group calls on FaceTime.
The flaw, first revealed by the 9to5Mac blog, appears to occur when both users are running version 12.1 of Apple's mobile operating system iOS, or newer. It also affects Mac users when they are called from an iPhone.
The technique involves using the software's group chat function, apparently confusing the software into activating the target's microphone, even if the call has not been accepted.
The eavesdropping ends when the call is cut after too many rings.