Specifically, the promotion is for up to $750 off the iPhone XR over 36 monthly bill credits at $20.84 per month. You can also put the $750 credit towards an iPhone XS or iPhone XS Max to reduce the price of the smartphones.
New customers will need to activate two new lines of qualifying service, one of which needs to be used with the new device being purchased. Existing T-Mobile customers will need to activate at least one new line of qualifying service to be used with the new device.
Secondly, you'll need to trade in one of the following eligible devices in good condition: iPhone X, iPhone 8, iPhone 8 Plus, iPhone 7, iPhone 7 Plus, iPhone 6s, iPhone 6s Plus, iPhone 6, or iPhone 6 Plus.
T-Mobile notes that this offer is available both in store nationwide and online at T-Mobile's website, but it'll only last for a limited time.