Well, this is alarming. You have probably already heard about this, but in case you haven’t… TURN OFF FACETIME ON ALL YOUR DEVICES NOW.
It turns out that anyone can call you on your iPhone and, before you answer, simply swipe up from the bottom of the screen and add your number to a Facetime group. From then on, your iPhone will start transmitting audio to this person, even if you didn’t answer the call. And even worse! If you press the power button on your iPhone you will start transmitting video as well.
I have now disabled Facetime, which I never use anyhow, on my iPhone and iPad. This also affects Mac computers.