This sign is located at one of AT&T's Long Lines facilities, a nuclear-hardened communications network comprised of microwave towers and buried coaxial cable that span the United States.
Might this sign indicate the presence of the coaxial cable below? Or maybe newer, higher bandwidth fiber optics that serve the same purpose/client as the original? You could ask AT&T, but if they told you, they'd have to kill you.
(But you can find some information here!)