I'm looking to get my grandpa an iPhone 7. He has MetroPCS as a carrier and I've been looking at A1660 models since these are the ones that are unlocked, however a seller on eBay states theirs is tied to "Straight Talk." What does this mean? Does it mean it can only work on a "Straight Talk" company phone plan? Or are all iPhone 7 A1160 models indeed unlocked and ready to work on any carrier?