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