simplex is a relatively new chat app but it's evolving pretty fast, i suggest it too, but it has several bug and missing feature that need to be fixed and added in order to make it like a standard chat application
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the call will close if you go back to the chat (they should fix that in the next version)
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it does not ring while you call
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it can only be synced to a one desktop via the wifi
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it has only local notification system (due to security reasons)
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the calls are pretty susceptible to the line stability
but seems the only real secure chat app out there, you can check out the roadmap in the description of GitHub
it's not centralized, you can choose your smp, xftp and webrtc ice servers, or you can install them in your own server
so is a decentralized chat application
so far i was able to do calls and videocalls to everyone (i have 8 friends there), but i won't say that the calls are stable, the application needs time to stabilize, and that's what they are doing, you can check the the GitHub page or chat with the developer if you want to know more