XMPP. I think @ron282 project to port Kaidan to SailfishOS GitHub - ron282/harbour-kaidan: Simple and user-friendly Jabber/XMPP client for every device and platform is very good. Recently, Kaidan developers commented in the Kaidan XMPP support room that they are working to bring multi-user rooms in the next version and more future features like calls and video calls, so SailKaidan can take advantage of it and finally have a good XMPP client in SailfishOS.
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