as per any standard , it might be positive ground. As per Telecom bibles, a positive grounded supply on the olden days open wire lines, helps to save copper wires from electrolytic action between the ground and the other wire, atmosphere acting as electrolyte.
Earlier designs have slowly disintegrated copper wires. then the technology had been changed to positive ground. most telecom installations across world follow this as a principle, including AT&T and many other American digital switches.