The PSTN and similar systems do target five-9s, but fortunately that only requires keeping it to ~20 minutes downtime over 4 years. ~20 minutes over 40 years would be six-9s.

    (* 1e-5 365.2425 24 60 4) => 21.038
    (* 1e-6 365.2425 24 60 40) => 21.038