At some point the four largest cellphone carriers will start to lose money on their Apple Inc. (NASDAQ: AAPL) iPhone 6s sales. For T-Mobile US Inc. (NYSE: TMUS) that day may have already come. It offers an iPhone 6s for $5 a month.

The deal does demand a trade-in, which can include earlier versions of the iPhone and some Samsung products. After the trade in, the offer gets even better:

Starting 9/25, you can switch to the latest iPhone for $0 upfront and just $5 per month with JUMP! On Demand. Plus, upgrade your phone up to three times per year with no waiting period or upgrade fees.

Zero upfront. Zero waiting. It’s the perfect way to upgrade to the latest and greatest iPhone

The iPhone 6s has 16GB, and the customer does need to sign for an 18-month carrier plan.

The smartphone prices wars, which include ever more favorable carrier fees, are not new. The level of aggressiveness is. The market share advantages of adding new customers may last for years, if those customers remain loyal. With each new and aggressive plan, customer loyalty may erode. T-Mobile has a great deal at stake. It claims it can take share from market leaders AT&T Inc. (NYSE: T) and Verizon Communications Inc. (NYSE: VZ). It will have to do so to be a financial success.

So far, Wall Street favors the T-Mobile approach. The company’s stock trades near a 52-week high at $43, against a range for the period from $24.26 to $43.43. By contrast, the other carrier that trails Verizon and AT&T by a great deal, Sprint Corp. (NYSE: S), trades toward the lower end of its range, at $4.34 against a 52-week spread of $3.10 to $6.78.

While customers enjoy all the benefits, investors may not. Giving things away, even in great numbers, may be the downfall of T-Mobile yet.

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