AT&T Inc. will let customers trade in their old iPhone for a new iPhone 6 and as much as a $300 credit, joining the wave of deals across the industry designed to lure more subscribers.

The offer will start Sept. 12, the same day Apple Inc.’s new devices will be available for pre-order, AT&T spokesman Fletcher Cook said. The phones that are traded in have to be in good working order to qualify, he said. AT&T is now the last of the major four national wireless providers to announce a promotion this week targeted at winning iPhone customers.

Each year, carriers count on new iPhones to help gain market share and data-hungry smartphone users as overall growth in wireless customers slows. After Apple unveiled the iPhone 6 yesterday, Sprint Corp. announced the “iPhone for Life” plan, which for $70 a month lets users rent the iPhone 6 and upgrade to a new version every two years. Verizon Communications Inc. is offering a $200 gift to customers who trade in older models and sign up for a new two-year contract, and T-Mobile US Inc. says it will top the best trade-in price with an added $50.

AT&T already offers two deals that also will be extended to the new versions of the iPhone: get a $100 credit on your bill when you purchase any iPhone and add a new phone line to an AT&T Next account; or buy an iPhone for no money down with AT&T Next and get a $200 discount on any iPad.