Apple has introduced the iPhone Duo, its first foldable iPhone, with a passport-like design that opens like a book. The device starts at $1,999 and expands from a 5.4-inch outer display to a 7.6-inch screen for video, gaming, work and multitasking. AT&T CEO John Stankey said foldable phones have already attracted a dedicated audience, particularly among Android users, but remain a niche product rather than a broadly adopted category. He said Apple could alter that pattern among iOS users, although he currently expects demand to remain concentrated among customers seeking a larger device and willing to pay more. Stankey said he might consider buying the Duo if it could replace the need to carry an iPad.