I think in most cases its just a couple of agents with an office on site.
Pre 911 it could mean that there was as little as a direct phone line to Customs available for reporting your entry. Pick the handset up, no dialing required, and INS answered eventually.
Back when things were more civilized we could enter back into the states from the islands with nothing more than a phone call within 24 hours of arrival provided we weren't carrying any non-citizens aboard. Trans border vessels on the great lakes could obtain "passes" through registration that allowed unlimited crossings with no