The company said on Wednesday that when it begins selling the iPhone 5 on Friday, it will offer a discount for people who trade in their older iPhones. Customers can trade in an iPhone 4S or iPhone 4, for example, for the iPhone 5 at no cost up front, along with a $120 credit toward the monthly device fee. The offer lasts through mid-June.

T-Mobile is doing its best to lure away customers from rival networks.