T-Mobile US Inc. said it will beat other wireless companies’ offers for smartphone trade-ins, a move designed to lure customers who want to exchange their phones for devices like Apple Inc.’s new iPhone.

T-Mobile will track the market for trade-ins to make sure its prices are better, it said today in a statement. If a customer finds a better offer from a national competitor, T- Mobile will top it and give the user an additional $50.

The limited-time offer starts Sept. 17, just in time for the debut of the next iPhone. Apple Chief Executive Officer Tim Cook will unveil the company’s latest iPhones and a wearable gadget at an event tomorrow. The iPhones will have 4.7-inch and 5.5-inch screens and will include software that lets the handset act as a mobile wallet, while the watch-like wearable device is expected to include features for tracking health and fitness activity, people familiar with the plans have said.

“We’re going to see some of the most phenomenal devices ever in the coming months, and that means a whole lot of Americans trading in their devices and upgrading,” T-Mobile CEO John Legere said in the statement.