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Illinois Bell was a local operator of the Bell Corporation starting in 1877. In 1984, the Bell Corporation was determined to be in violation of antitrust laws, and was ordered to break up into seven, "Baby Bells," including Ameritech, which inherited the Illinois Bell Company. Illinois Bell kept its name until the 1990s when Ameritech changed it for marketing purposes. Several of the baby bell companies have gone on to become major phone providers in the US, such as AT&T and Verizon.
Tunstall, Brooke (1985). Disconnecting Parties: Managing the Bell System Break-Up, an Inside View. New York: McGraw-Hill.