Although T-Mobile hasn't announced an end-date for the promotion, it's likely not going to last too far into the new year. As is currently stands, you can get either the iPhone 8 or the iPhone XR for free with the current promo. With more expensive phones like the iPhone 8 Plus, iPhone X, iPhone XS or iPhone XS Max, you'll get a maximum of $750 off the purchase price.
For example, you can get an iPhone XS for $999 minus $750 in monthly bill credits bringing your total outlay to $249. In order to get the full $750 credit, you'll need to trade in any previous iPhone flagship ranging from the iPhone 6s/iPhone 6s Plus on up through the iPhone X in good condition.
One other condition is that you will need to purchase the new phone using T-Mobile's Equipment Installment Plan (EIP). We should mention that you will pay the full upfront cost of the iPhone at first, then will be reimbursed in the form of equal monthly credits either over the course of 24 months or 36 months.
You'll receive text message confirmation once you're qualified for the deal, and your 24 monthly credits will begin within two bill cycles after purchase. However, if you're well qualified and purchase certain devices, you could receive an extended 36 monthly credits instead.
Finally, to take part in this promo is that new customers will have to open two voice lines to qualify, while existing customers will just need to add one new line.