T-Mobile is currently offering a large sale on smartphones for back-to-school. Headlined by buy-one-get-one FREE iPhone X/8/7/Plus when porting in a new line of service and trading in a device, you can save up to $700 with this offer. Starting next week, T-Mobile will also be discounting the iPhone 6S to under $100 (with a total discount of over $300), but specific pricing is yet-to-be-announced.