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.