Jami on desktop, not on phones yet:
Available only for desktop users for now, the new Push-to-talk feature offers a new effortless way to communicate: simply press a button for hands-free, instant, and convenient audio messaging. It’s like in the olden days of gaming when gamers would key bind the Push to talk feature to be able to talk when necessary.
So jami all the way, 🙂
This is a security feature. Other communication mechanisms having the keys somewhere else not owned by you is rather something I wouldn’t stand. And to me it’s unsafe that messages would be kept way long on the servers.
On xmpp the sync happens from server to all syncing clients, and the proper XEPs need to supported in both the client and the server.