In what area of the united states was the concentration of telephone exchanges the heaviest?

Telephone exchanges are telephone system s located in central offices over a given geographical area to provide connection to connect calls that made it possible to make calls at home, business, and public spaces. Due to a rapid expansion of telephone network in the United States in the 20th Century, the Bell system was introduced by its executives as a new way of dialing the phone. Before human operators, mostly women directed the call to their destinations, but digital systems could now establish calls between users. In 1879 the first telephone exchange in Atlanta was opened called the National Bell Telephone Company located at Kimball house and Pryor streets.
The dialing systems used alphanumerical addresses named after phone exchanges