An Apple software problem allows users to have FaceTime audio-video calls to hear or see the person they are calling without having to accept the call.
The problem was noticed by many users who use the latest version of iOS, 12.1. The ability to listen to another person is activated when you scroll up to add another number to the conversation and add your phone number. So the person who started the call is able to hear or see the other person. Meanwhile, no sign is shown on the screen.
An Apple representative said that "the company is aware of the problem and is working to resolve it".
To prevent someone from hearing or seeing you without your approval, go to Settings -> FaceTime and unlock the green button until the problem is resolved by Apple.