A flagrant bug in Apple's FaceTime According to the 9to5Mac software, users can listen to other people while a call is ringing.
The error works by simply selecting another user and then inserting the source number from the Add Person Screen before answering. Apparently FaceTime seems to believe that a Group FaceTime conference call is taking place, transmitting audio from the recipient's device, regardless of whether the call was accepted or denied.
Gizmodo was able to easily replicate the error in a few seconds. Follow these steps.
According to Verge, this very serious security problem is complemented by another problem: when the "conference call" takes place, the receiver presses the power button or the volume key to ignore the call audio as well as video from the front camera, also without visual notification (in this case, however, the audio of the caller is also activated). This bug was also confirmed by Mashable and Buzzfeed News, the latter finding that Do Not Disturb seems to at least block access to the microphone.
At 9to5Mac, the problem is even more serious on Mac computers:
We also have replicated the problem with an iPhone calling a Mac. By default, the Mac rings longer than a phone, so it can act as a bug longer.
Apple told BuzzFeed News in a statement that it "knows this problem and we have found a solution that will do this." This software will be released later this week in a software update. "However, it appears that any FaceTime user with a device that supports Group FaceTime (iOS 12.1 / MacOS Mojave 10.14.1 or later) may be able to send audio or video streams to all users without their knowledge – and The steps required to do this are both trivial and up-to-date on the Internet – everyone should definitely disable FaceTime until Apple fixes the problem.
Simply navigate to the Settings application on an iOS device and turn the FaceTime icon switch gray. On a Mac, open the FaceTime app, then click FaceTime on the menu bar and choose Turn Off FaceTime.
Update: 1/28/2019 at 11:15. ET: According to by Bloomberg's Mark Gurman it looks like Apple will disable the Group FaceTime features. 
[9to5Mac/The Verge/BuzzFeed News]