Illinois Bell Telephone Company, LLC is the Bell Operating Company serving Illinois. It is owned by AT&T through AT&T Teleholdings, formerly Ameritech. Their headquarters are at 225 West Randolph St., Chicago, IL. After the 1984 Bell System Divestiture, Illinois Bell became a part of Ameritech, one of the 7 original Regional Bell Operating Companies. The Illinois Bell name continued to be used until January 1993, when Ameritech dropped all of their individual Bell Operating Company names in favor of using their corporate name, Ameritech, for marketing purposes. Illinois Bell started doing business as Ameritech Illinois.