AT&T is the exclusive U.S. carrier for the new smartphone, which will be available in matte black and matte white. AT&T currently offers the largest selection of Windows Phone devices of any operator in the U.S all connected to the nation's most reliable 4G LTE network.2 Lumia 640 XL Features With the large 5.7 inch HD display and built-in Microsoft Office, you can read your emails, edit documents, update spreadsheets or review presentations while on-the-go.