@wjmaggos @railmeat @briankrebs @signalapp @matrix or alternatively #xmpp / @snikket_im which has OMEMO that's based on the same double-ratchet algorithm as Signal. It also bridges well to SMS and voice phones using jmp.chat (or similar Vonage/Twilio for non-US/Canada numbers)

Matrix has some nice features that XMPP lacks, but XMPP is still a well used and reasonable alternative.