AT&T Inc. began its history as Southwestern Bell Telephone Company, which was a subsidiary of the Bell Telephone Company founded by Alexander Graham Bell in 1880. Bell Telephone Company evolved into American Telephone and Telegraph Company in 1885 which had since rebranded to AT&T Corporation.
AT&T established a network of subsidiaries in the United States and Canada that held a government-authorized phone service monopoly, formalized with the Kingsbury Commitment, throughout most of the twentieth century. This monopoly was known as the Bell System, and during this period, AT&T was also known by the nickname Ma Bell. For periods of time, the former AT&T was the world's largest phone company.
According to wikipedia