The YouTube channel The Grate Debate has figured out how to consistently trigger the teaser, and I wanted to confirm their methods having tested out multiple theories. Beware of puzzle spoilers below.
Using the method in the video, I was able to trigger the "second baby laugh" every time, which leads to the ending. Note that you will need a microphone.
Here's what you do when you reach the final room:
1) Look at the clock near the start until you hear 12 chimes. Wait for the "haunting" that happens afterward to completely end. Walk approximately 10 steps until you hear the first baby laugh. You can walk a little farther if you need to. To be clear, a "haunting" is when you hear gibberish, crying, screeching, or scratching noises.
2) Wait for the second "haunting." After the ghost is done crying, but before the haunting ends, whisper "Jarith" into a PS4 microphone. Try whispering it multiple times, yelling it into the microphone, and switching your mute button on/off just to make sure it "takes." You can say it as often as you want, but the interaction is delayed. Many people are reporting that any "J" name works, but Jarith worked every single time for me, so take that as you will.
3) Do not move at all. Stay completely still even after you hear the subsequent second baby laugh -- keep standing still as your controller vibrates until you hear the phone ring. If you move, you'll have to reset the loop again by going through the last door.
4) As the phone is ringing, hold R3 to focus on it until you hear "you have been chosen." Walk out the final door as normal.
This isn't the only method, but it's the most consistent -- though it does require a microphone. You can trigger the first and second laughs interchangeably, but I prefer this particular order. Again, there's still talk over whether or not this method works (with the video author even asking for confirmation), so I wanted to set the record straight having tested it myself.
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