So I want: - multiuser chat (persistent) - voice-calls (let's have them 1on1) - file-transfer - concurrent clients

Which mature clients and servers should I choose from, implementing all this (basic) stuff?

Let's assume this hypothetical client doesn't exist (I'm not actually sure if it does or not): are you suggesting it's better to start over and create an entirely new chat protocol instead of just writing a client that has the features you want? These features do all exist within existing open protocols, and clients do exist that implement some of these features (and probably all, but I'm not sure). So I don't see any reason to start over.

I've been using [Wire](https://wire.com/en/) very happily for some time now. Great interface, top quality audio/video, good chat, persistent, very secure, open source. I'm surprised it hasn't received more mentions.

Yeah. Did you actually try calling someone with any 2 of these clients?

All things you need will be covered by Xabber in second half of 2019, on ios, web, android.

promised for 6 years... and remember this thing is commercial, not OS...: "Nonsense comments from clueless commentors. It's not our problem if you didn't update on feb14. Also, it's our app and we do whatever we want, please uninstall Xabber and use something else. You had no voice or rights here besides those that have been generously given by us. Now they are revoked."