Sprint Corp announced a $1 per month plan for Apple Inc's new iPhone, upping the ante in a battle among top U.S. carriers to sign up customers ahead of the phone's launch on Friday. Sprint said on Thursday customers can get a 16 GB iPhone 6s for $1 per month and an iPhone 6s Plus for $5 a month, when trading in an iPhone 6. Rival T-Mobile US Inc is offering the 16 GB iPhone 6s for $5 per month in exchange for an iPhone 6, 6 Plus or Samsung Electronics Co Ltd's Galaxy Note 5 and Galaxy S6 versions.