Who's knocking?
You knock first. Tap start a hello and hold your phone up - the neighbor switches to I'm answering, scans it, and shows you their answer.
Their start a hello screen is already up. Scan it, or paste what they copied, then tap connect.
The honest fine print of a hello
How it stays direct. The greeting itself is WebRTC: encrypted between your two phones, phone to phone. The scan you just passed is the signaling - it only carries the opening words of the connection, and it went through your hands, not a server. That's the whole trick: a doorway you pass in person beats a company you pass in packets.
One quiet helper. Your phone asks a public address guide (STUN) where its own front door is. The guide sees the door number, never the letters inside, and it is not in the path of your messages.
What it can't do. This hello is real time - it lasts while you're both here, and it doesn't record or wake up later. Under a rare strict Wi-Fi, a direct hello can fail; it fails honestly rather than secretly rerouting through a relay. For "leave a message," a building keeps a shared page instead - that's the union book.