Here's the Real Reason Investors Should Be Worried About Apple's iPhone 5

Updated to include UBS analyst comments

NEW YORK (TheStreet) -- In the daily speculation on everything Apple(AAPL) , investors may be missing what could be the biggest story to emerge about the iPhone 5 after its September release.

Apple's newest iPhone and a lineup of tablet computers will provide a fourth-quarter test to the sustainability of profit margins for telecom giants Verizon(VZ) , AT&T(T) and Sprint(S) .

A recent Wall Street Journal report indicates that Apple may be cutting orders with suppliers who build the iPhone 5, potentially indicating less-than-anticipated demand for the company's newest smarthphone. The report, which cites anonymous sources, adds to a frenetic 2013 in the Apple rumor mill.

Previous media reports indicated Apple might start selling iPhones as low as $99 by year-end. However, Phil Schiller, Apple's head of marketing, quickly nixed such speculation.

Apple, at its 11 price-to-trailing-earnings multiple, seems to attract an ever-growing number of bulls who see America's most profitable company as an obvious 2013 value -- and skeptics who call the company a value trap and foresee a commoditization of its premium-priced tech products.

In the interim, the more urgent iPhone 5 story may have little to do with Apple or its shares.

As telecom giants Verizon and AT&T prepare to report fourth-quarter earnings, analysts are bracing for the full impact of smartphone subsidies and, in particular, the earnings hit that might come from an entire quarter worth of subsidized iPhone 5 smartphone sales.

Carriers already have indicated that Apple's newest smartphone will have an impact on earnings, even if trends like underlying subscriber and average-revenue-per-user growth indicate a strong outlook for the industry's top players.