Duetto

Duetto is a tiny "Discord" made to measure for two people only. It is not an app for calling: it is a permanent channel. Open the app and you are in; if the other person is there too you connect by yourselves, and otherwise you stay reachable and are told the moment they arrive.

Audio and video travel end-to-end encrypted straight between the two phones. The server - yours, or one somebody let you in to - is only there to let you find each other, and cannot read anything. No account, no phone number, no service of anybody's.

• One tap and you are in the channel, and you stay: the app survives the night, the pocket, the change of network.
• Video when you want it, with the quality chosen for both phones.
• The other person can call you back with a sound that gets through even with the ringer down.
• Pairing with an eight-digit code or a QR code held up to the other phone; invitations for whoever is to open connections of their own on your server.
• Silent during a phone call, back when it ends.
• Free software, GPL-3, and a server you run yourself: a small Node.js process and coturn.

You need Android 7 or later, and a server: the README says how to put one up in a quarter of an hour.