AT&T Inc. AT&T Inc. is an American multinational conglomerate holding company headquartered at Whitacre Tower in Downtown Dallas , Texas . It is the world's largest telecommunications company, the second largest provider of mobile telephone services, and the largest provider of fixed telephone services in the United States through AT&T Communications . Since June 14, 2018, it is also the parent company of mass media conglomerate WarnerMedia, making it the world's largest media and entertainment company in terms of revenue. As of 2018, AT&T is ranked #9 on the Fortune 500 rankings of the largest United States corporations by total revenue. AT&T Inc. AT&T's corporate headquarters in Dallas , Te...