AT&T is the second-largest mobile carrier in the United States with over 100 million wireless subscribers. The wireless carrier is a subsidiary of the larger company AT&T, which operates wireline internet, TV and phone services across the U.S. AT&T has played an important role in the Windows Phone scene in the United States, with the carrier usually having a choice selection of high-end exclusives.
As of September 2014, you can find devices like the Lumia 1520, Lumia 635 and Lumia 1020 at AT&T. The Lumia 520 can be picked up as an AT&T GoPhone. You can look forward to the HTC One M8 for Windows and Lumia 830 joining AT&T sometime before the end of the year.
AT&T operates its network with GSM and LTE technology for both voice and data, primarily on 700, 850, 1700 and 1900MHz spectrum. In early 2014, AT&T also announced VoLTE (Voice Over LTE) service and compatible devices, though it will be some time before it becomes a mainstream service.