Telegram. It's open source, uses end to end encryption, and, unlike whatsapp, supports multiple connected clients at a time - including desktop clients for all platforms.
It's public domain, not open source.
End-to-end encryption is easy - you just need to send a courier with a one time pad.
And yes, there are telegraphs supporting multiple concurrent connections by using pitch shifting and filters so the receiver will only hear one set of beeps. But not more than a few.
Sure, there are desktop clients for all platforms - wooden, metal and marble top desktop can have clients, and there are even keys that mount on tilted desktops.
Of course you'll be hard pressed to find anyone on telegram
Indeed. Even Her Majesty The Queen stopped sending telegrams a few years ago. A shame, really.