focus_unavailable_for_client· reason_code· applies to service_callClient cannot use the committed `session_focus` (e.g. focus not in local `foci_preferred[]`, region restricted, capability mismatch). Client MAY fail closed without joining the call rather than silently degrading; clients MUST NOT pick a different focus to bypass `session_focus_no_split_brain`. See zh/crypto-media/media-service-binding.md §5.