Find your focus and avoid making phony offers.
If you’re going to enter a world run by giants, make sure what you offer is legitimate. The best competitive advantage you have over places like AT&T and T-Mobile is your ability to control price.
Maybe you focus on offering used and refurbished phones at a huge discount. Maybe you find a distributor who sells to you in bulk. Whatever it is, controlling your price points will help ensure that customers buy from you instead of the other folks.