UPDATE: Apple has turned off the group facetime function it might not working anymore. Some claim it still does. try it out. Here's a WAVY article with updated info.
The bug lets you call anyone with FaceTime, and immediately hear the audio coming from their phone before the person on the other end has accepted or rejected the incoming call.
Here’s how to do the iPhone FaceTime bug: (Source: 9to5mac.com)
- Start a FaceTime Video call with an iPhone contact.
- Whilst the call is dialling, swipe up from the bottom of the screen and tap Add Person.
- Add your own phone number in the Add Person screen.
- You will then start a group FaceTime call including yourself and the audio of the person you originally called, even if they haven’t accepted the call yet.