The game was online-multiplayer, where two players would work their way through a series of puzzle-like rooms together. But they would match you with a random person and you would have no idea who they were, not even a screen name.
Then, the only way you could communicate was by making a little chirping noise. Tap the button and you make a little chirp. Hold it down and release for a big chirp. If you weren't looking at the other player you would see a faint glow in their direction when they chirped, so they could grab your attention. And that was it.
There were times when I would miss something and start to continue forward, so the other player would chirp-chirp-chirp-chirp to say "Wait, look over here!" I had no idea if they were pissed at me or being friendly about it, but given how cute the whole thing was I would assume the latter. It just felt nice, like I was meeting a new friend every time.
I've gone back to that game several times over the years, but sadly nobody seems to be playing it anymore. My first time playing feels like a once-in-a-lifetime experience that can never be replicated. Really great game.