Cars. Houses. Contracted electronics. These are items for which the old "buy one, get one" scheme rarely works. But since T-Mobile has ostensibly given up cellular contracts, it makes a sort of sense... at least if you're buying phones for more than one person on the same plan. The carrier is now offering a buy one, get one 50% off discount on all versions of the Galaxy S6, Galaxy S6 Edge, Galaxy S6 Edge+, Galaxy Note 5, the LG G4, and the LG V10, as well as the latest two generations of iPhones.
All colors and capacities sold by T-Mobile are eligible, and you can purchase up to 12 phones at a time(!) and apply the discounts. Mixing and matching phone models is allowed, but only for one manufacturer - if you buy one Samsung phone, your "half off" phone has to be a Samsung, too. Naturally, you need to connect those purchases to existing lines with a post-paid plan or activate new ones for each phone - you can't just get a bunch of discounted phones and sell them on eBay.
Strangely, the offer can't be redeemed online, only in a T-Mobile retail store or over the phone. (Remember buying things on a phone line? With your voice? Talking to another human being? Crazy.) If you're buying on an installment plan, you may need to qualify with a credit check. T-Mobile's promotional page says that this offer is good for "a limited time only," but doesn't mention precisely when it will expire. The buy one, get one half off offer can be combined with trade-in credit and credit for switching from another carrier.