US carriers sure do love their Buy One, Get One free (BOGO) deals. The latest such offer comes from T-Mobile, and is a limited time Father's Day thing, so make sure you act fast if you find it interesting.
The Galaxy S8 and S8+, the LG G6 and V20 are all involved. If you snag any two of these devices, the one which is cheaper will be free. If you're already a T-Mobile customer, you'll need to add a line with unlimited data for this to work. If you aren't with the magenta carrier, then you'll need to open two new lines (on the One plan). In both cases, the handsets need to be purchased using the company's Equipment Installment Plan.
After you do all this, you have to submit your rebate request online, and then you'll receive a refund for the device of lesser value via a prepaid MasterCard. This should be at your doorstep within 8 weeks of the moment you make the claim.