Chicago Telephone Company
Chicago Telephone grew rapidly. By 1905 it operated 100,000 telephones in the city of Chicago alone. As a result of this growth, the firm built a new headquarters in 1912 at 212 W. Washington Street in Chicago, adjoining its former headquarters building on Franklin Street. The two buildings together constituted the largest telephone building in the world.
In 1920 the firm began to grow through acquisition, buying the Illinois properties of the Central Union Telephone Company. With this purchase and its already existing territory, Chicago Telephone operated in almost all of Illinois as well as Lake and Porter counties in Indiana. The company acknowledged its statewide presence by changing its name to Illinois Bell.
Both of these companies were pillars of the "Bell System," the national telephone network that was coordinated after 1900 by American Telephone and Telegraph (AT&T). The Bell System remained the backbone of the American communications infrastructure from the 1870s until the court-mandated breakup of AT&T in 1984. Following the Bell breakup, Illinois Bell became part of Ameritech, which, in 1999, was acquired by SBC Communications, a telecommunications firm based in San Antonio, Texas.