You can be sure that carriers will be trying hard to get your iPhone 6s purchase when the device launches on September 25th, but T-Mobile may be trying harder than most. It's launching a promotion that day which will drop the cost of an iPhone 6s when you trade in your old iPhone and go with the network's Jump On Demand upgrade program. If you hand in an iPhone 6, you'll pay as little as $5 per month for the new handset; turn in that 'old' iPhone 5s and you'll get the 6s for a still-reasonable $10 per month. Giving up any other device will lower the payments to $15 per month. You can pay more if you want something beyond the base model, of course. While you'll likely want to look at Apple's own installment plan if you insist on a factory-unlocked iPhone, the rate is low enough that it may well lure you away from Sprint and other providers with Apple-specific deals.