There's a FaceTime bug that lets you hear through someone else's iPhone, even if they haven't answered your phone call. It was previously reported on Twitter by user Benji Mobb and reported by 9to5Mac.
CNBC tested it and verified that it's a real bug. I tried placing a video call to my editor from my iPhone using Apple's FaceTime app. Then, before he picked up, I used the menu option to add somebody else to the call and, as 9to5Mac's directions state, I added myself to the call. Suddenly, I could hear my editor, even though he never picked up. We continued having a conversation while his phone only gave him the option to answer. There was no indication that the call had already gone through.
The Verge reports that, if the person you're calling tries to end it by tapping the power button on their phone, it sends video but no audio. That means the person calling you can see a video of you or your surroundings as if you'd answered the call. I replicated this with a friend.