The bug also allow some iPhone users to see through the camera of other iPhones.
Apple is working on a software fix which they expect to release later this week, according to CNBC.
Users who are concerned about their privacy are advised to disable the FaceTime option on their phones.
An Apple spokesman commented: “We’re aware of this issue and we have identified a fix that will be released in a software update later this week”.
The bug was first reported by technology website 9to5Mac on Monday.
It works on iPhones and iPads running iOS12.1 and Apple PCs running macOS Mojave which installed the group FaceTime feature recently.
The issue occurs when a user phones someone using FaceTime, then swipes to add another person to the call and adds their own number.
They can then hear the audio from the other phone even if the call isn’t picked up, without the other person being aware.
In some cases live video from the camera of the other phone is also displayed.
New York governor Andrew Cuomo issued a consumer alert for the product.
He commented: “The FaceTime bug is an egregious breach of privacy that puts New Yorkers at risk.
“In New York, we take consumer rights very seriously and I am deeply concerned by this irresponsible bug that can be exploited for unscrupulous purposes.
“I advise New Yorkers to disable their FaceTime app until a fix is made available.”
To turn FaceTime off on a Mac open the FaceTime app, select “FaceTime” at the top of the screen then choose “Turn FaceTime Off”.
Alternatively on an iPhone or iPad select ’Settings’, then ‘FaceTime’ after which you can turn off the green toggle at the top of the screen.
The Electronic Frontier Foundation’s Director of Cybersecurity stated: “The FaceTime bug works in both iOS and MacOC.
“Now would be a good time to disable FaceTime on everything.”
Meanwhile Wall Street Journal reporter Michael Derby added: “This is one of the worst Apple bugs I’ve ever heard of. Nasty.”
A number of Apple users took to social media to express their shock and concern at the development.
More to follow…