AT&T Inc. is an American multinational telecommunications holding company that is Delaware-registered but headquartered at Whitacre Tower in Downtown Dallas, Texas. It is the world's largest telecommunications company and the largest provider of mobile telephone services in the U.S. As of 2020, AT&T was ranked 9th on the Fortune 500 rankings of the largest United States corporations, with revenues of $181 billion. During most of the 20th century, AT&T had a monopoly on phone service in the United States. The company began its history as the American District Telegraph Company, formed in St. Louis in 1878. After expanding services to Arkansas, Kansas, Oklahoma and Texas, through a series of mergers, it became Southwestern Bell Telephone Company in 1920, which was then a subsidiary of American Telephone and Telegraph Company. The latter was a successor of the original Bell Telephone Company founded by Alexander Graham Bell in 1877. The American Bell Telephone Company formed the American Telephone and Telegraph Company subsidiary in 1885. In 1899, AT&T became the parent company after the American Bell Telephone Company sold its assets to its subsidiary. Wikipedia