Apple Inc. CEO Tim Cook’s decision to call out AT&T and other U.S. carriers for making iPhone users wait longer for discounted device upgrades, as he did on the company’s quarterly earnings call this week, was a brave one.

Whether his public comments help Apple AAPL, -3.17% in negotiations over future wireless subsidies remains to be seen, however. Read: Apple gives worst performance in a year.

Those negotiations will help determine how large a slice of U.S smartphone and tablet sales goes to Apple and how much goes to its rivals and carrier partners.

“Some carriers changed their upgrade policy,” Cook said on the call, after revealing that the company’s iPhone sales contracted in the U.S. during the quarter ended in December.

The carriers “stretched the time out to 24 months,” before allowing users to get newer iPhones either for free or at nominal cost, whereas many users previously received subsidized devices just 20 months into their service contracts.

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The hardball tactic by the wireless giants may help explain why Apple’s revenue forecast for the current quarter was much weaker than Wall Street expected.

iPhone sales comprised 56% of Apple’s sales in the December quarter.

Cook’s comments signal that Apple may be losing ground in negotiations with AT&T T, -0.48% and Verizon VZ, -0.39% over the payments that have subsidized iPhone growth for more than six years.

Both carriers now have other high-end smartphone devices to offer customers, offerings they didn’t have when the iPhone was introduced in June, 2007.

Apple’s innovation lead over Samsung 005930, -0.33% and others was wider then, which made it easier for former CEO Steve Jobs to extract lucrative subsidies that helped finance early iPhone sales, first from AT&T and later from Verizon and other wireless operators.

Yet Android phone makers can now offer a mobile browsing experience that’s good enough for most smartphone users, a market reality that’s weakened Cook’s hand in the negotiations, judging by his comments.

Apple, of course, has its own cards to play in subsidy negotiations covering the U.S. mobile market, where Apple smartphones and tablets are the leading driver of mobile commerce, according to Cook.

The CEO quoted comScore research figures indicating that iPhone users conduct 54% of U.S. smartphone browsing and 78% of tablet browsing.

That gives the company an opportunity in the mobile advertising market, which Google GOOG, -2.37% now leads thanks to its Android platform, and where Facebook FB, -0.89% and Twitter TWTR, +2.03% are growing ad sales rapidly. Read: Facebook and Twitter make this a banner year for ad blockers.

The size of Apple’s opportunity will depend mostly on how may iPads and iPhones the company can sell, however, as mobile device growth shifts from smartphones to tablets.

The company said iPhone year-over-year sales growth fell to under 7% in the fiscal first quarter, while iPad sales grew 12%.

If the iPad’s market share remains large enough to attract the most innovative apps from developers, that in turn will attract more mobile users and advertisers to the iOS platform.

Still, that business is a small fraction of Apple’s hardware business, which means more ad sales won’t save Apple’s stock price if iPhone sales continue to weaken.

The company needs to either win the product transition from smartphones to tablets over Android tablets or roll out some other device that captures the next burst of growth in the mobile handheld market.

If it doesn’t, Cook’s complaints may not help Apple get more subsidies to finance future iPhone and iPad sales.

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