WordNet (r) 3.0 (2006):

central office
    n 1: (usually plural) the office that serves as the
         administrative center of an enterprise; "many companies
         have their headquarters in New York" [syn: headquarters,
         central office, main office, home office, home
         base]

The Free On-line Dictionary of Computing (30 December 2018):

central office

    The place where telephone companies terminate
   customer lines and locate switching equipment to interconnect
   those lines with other networks.

   (1995-03-20)