AT&T Inc. (American Telephone and Telegraph Company) is a multinational telecommunications holding company headquartered at Whitacre Tower in Downtown Dallas, Texas. It is the largest wireless carrier in the United States and the world’s fourth-largest telecommunications company by revenue. As of 2023, AT&T was ranked 13th on the Fortune 500 rankings of the largest United States corporations, with revenues of $120.7 billion. During most of the 20th century, AT&T had a monopoly on phone service in the United States. The company was founded as the American District Telegraph Company in St. Louis in 1878 and expanded its services to Arkansas, Kansas, Oklahoma, and Texas through a series of mergers.