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.

AT&T was founded as Bell Telephone Company by Alexander Graham Bell, Thomas Watson, Gardiner Greene Hubbard, and Thomas Sanders after Bell's patenting of the telephone in 1875.

The company is headquartered at Whitacre Tower in downtown Dallas, Texas. On June 27, 2008, AT&T announced that it would move its corporate headquarters from downtown San Antonio to One AT&T Plaza in downtown Dallas. The company said that it moved to gain better access to its customers and operations throughout the world, and to the key technology partners, suppliers, innovation and human resources needed as it continues to grow, domestically and internationally.

As of 2019, AT&T is the world's largest telecommunications company. AT&T is also the largest provider of mobile telephone  services and the largest provider of fixed telephone services in the United States. It had 181.2 billion USD of revenue and 247.8 thousands employees.

In May 2021, AT&T announced plans to spin off WarnerMedia, which will merge with Discovery, Inc. for $43 billion.

According to wikipedia.org