AT&T

AT&T Inc. is an American multinational conglomerate holding company that is Delaware-registered but headquartered at Whitacre Tower in Downtown Dallas, Texas. It is the world's largest telecommunications company, it is also 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.

Source: Wikipedia