Perhaps fearing the rapid, recent growth of T-Mobile (TMUS 0.92%), AT&T (T 1.10%) has unveiled a new plan that could lead to the carrier selling fewer of Apple's (AAPL -1.22%) iPhones. Starting next week, AT&T subscribers will have the option of signing up for the "Mobile Share Value Plan" -- a plan that would separate the cost of their smartphone from the cost of their service. Subscribers who pay for their phone up front, activate an old phone they already own, or pay for their phone in monthly installments, will receive a discount on their monthly bill.

Among subscribers that opt for this plan, cheaper handsets powered by Google's (GOOGL -1.23%) Android could become far more attractive. AT&T subscribers who switch might be enticed to pick up a less expensive Android phone rather than a more expensive iPhone.

T-Mobile gets fewer iPhone activations than the other carriers
Last quarter, T-Mobile sold (as a percentage of total smartphone sales) far fewer of Apple's iPhones than the other major carriers. In total, just 21% of the smartphones T-Mobile sold were iPhones, while more than 50% of Verizon's smartphone sales were Apple-made handsets. Unfortunately, AT&T has stopped disclosing iPhone sales data, but historically, about 80% of the carrier's smartphone sales have come from Apple's iPhones, and though that percentage may have declined somewhat, it's still more than likely far higher than T-Mobile's.

There's a variety of reasons to explain the discrepancy between T-Mobile and the other carriers. For starters, T-Mobile only got Apple's iPhone this year -- its longtime subscribers may have been locked into Google's competing Android ecosystem, while its employees may have been more comfortable selling devices powered by Google's mobile operating system.

But a larger factor may be T-Mobile's new pricing policies. Starting this year, T-Mobile opted to do away with phone subsidies entirely, forcing subscribers to buy their handsets up front, or pay for them in monthly installments. This has the obvious effect of making cheaper handsets more enticing, as monthly costs can vary significantly based on the actual price of the phone. T-Mobile subscribers opting for Apple's iPhone 5s, for example, must pay $25 per month for 24 months, in addition to a $50 down payment. In contrast, the Android-powered LG Optimus F6 can be had for just $12 per month with no down payment.

Now, consider the alternative: a standard, two-year contract with AT&T. No matter which phone a subscriber chooses, their monthly bill will be the same -- the only thing that varies is the up-front cost. Picking a cheap handset powered by Google's Android might save an AT&T subscriber a couple hundred dollars up front, but it's hardly worth it -- they're stuck with the phone for two years anyway, so most of them logically choose the higher-quality Apple-made device.

Apple's iPhone business is dependent on subsidies
But that wouldn't be the case if AT&T wasn't subsidizing Apple's iPhone: It's overwhelming clear that Apple depends on carrier subsidies for its iPhone sales. In emerging markets, where most carriers don't subsidize phones, Google's Android dominates. Where phone subsidies are relatively generous (like in the U.S. or Japan), Apple accounts for a large portion of the market.

That's why this trend toward reduced carrier subsidies should be worrying for Apple. Despite T-Mobile's recent growth, it remains one of the nation's smallest carriers -- but AT&T is the second largest. AT&T isn't abolishing subsidies entirely, but its new plan gives subscribers the option of forgoing subsidies in favor of reduced monthly bills.

If many of AT&T's subscribers take the company up on the offer, or if the trend away from smartphone subsidies continues, Apple's iPhone business could come under pressure in the coming quarters.