FaceTime bug lets iPhone users eavesdrop on persons they're calling
UPDATE: Apple has temporarily taken down FaceTime until it fixes the issue. You are no longer required to disable the feature from the Setting menu since FaceTime is now offline. An update that should address the privacy problem is scheduled for later this week.
Of course, that person can hear you as well, but only if it's aware that a FaceTime call is in progress. Even so, the person called will definitely be caught off-guard whenever it receives a FaceTime call, so the best way to prevent that from happening is to completely disable the feature until Apple fixes it “later this week.”
In order to trigger the FaceTime bug, you have to start a call, add a person, and choose your own phone number when prompted, thus you'll start a group call even if the other person's phone is still ringing.
The issue affects any pair of iPhones running iOS 12.1 or later, but also calls made to Macs using Mojave. You can disable FaceTime on your iPhone by heading to Settings and toggle the FaceTime icon to gray.
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Things that are NOT allowed: