Users can trade old iPhones of various storage variants. Whitney reports that carriers also offer their own trade-in options, though many come with conditions like the activation of a new account, or apply the trade-in value as a credit toward your bill.
T-Mobile is going the Gazelle route and offering to beat the other guys: If customers find a better trade-in value from the other three major carriers, T-Mobile will beat the offer and throw in another $50 toward the cost of a new device.