The offer works by buying a qualifying iPhone and getting a second iPhone half off via rebate on a Prepaid MasterCard® card. Customers must add a new line of qualifying postpaid service to their account, and one of the two new devices must be on this new line. New and existing customers are eligible, and any Simple Choice postpaid rate plan will qualify. Customers can get up to 2 rebates per account, and the device which receives a half-off discount is the one with equal or lesser value to the one you've ordered.
The full list of participating locations and other terms you might want to become acquainted with are available in the source link below.