Apple Inc's. debut of the newest iPhone on all four top U.S. carriers at the same time will test the loyalty of customers at AT&T. and Verizon.
The latest version, set to be unveiled tomorrow, will be the first to come out on T-Mobile simultaneously with its bigger rivals. Adding to the competition, Apple will introduce a cheaper iPhone along with an upgrade of the iPhone 5 tomorrow. AT&T had exclusive rights to the iPhone until 2011.