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It will be a secondary error resulting from original unreachability problem.
It means that you have tried to do something on a call that no longer exists on the other side, e.g. you lost connectivity; the other side hung up, but the BYE never reached you; the other side gave up trying; connectivity was regained; you tried to hang up.
It could also happen if the other side was reset during a call and didn’t close the calls down cleanly.