A massive surge in subscribers is slowing, and users are gobbling up more costly data. But that hasn't meant withering performance by mobile carriers like AT&T and Verizon.

By Kevin Kelleher, contributor

FORTUNE -- You might think life for mobile carriers these days is tough. After all, subsidizing the cost of smartphones to the tune of $400 or so per device is a drag on profits. There are also signs that mobile subscriber growth is in a lull, thanks to a sluggish global economy. And with more wireless subscription plans in the U.S. than there are people, the market is surely approaching total saturation.

Investors have sympathized. In fact, many of these concerns have been understood -- and accounted for -- by investors for months, even as subscriber growth was essentially flat in the first quarter of this year. Still, why are the big telecom stocks rallying this spring? Over the past two months, while the S&P 500 index has declined 4%, AT&T (T) has risen 15% and Verizon (VZ) has gained 16%. Even Sprint (S), with its smaller market share, is up 6% in the period.

Why are telecom carriers -- often seen as modern utilities, eking out modest growth in a competitive environment -- outperforming the broader market so strongly? It seems that they are preparing for an era of slower subscriber growth, scrambling for new sources of revenue from existing customers.

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Early efforts suggest that they are succeeding. Goldman Sachs (GS), for example, issued a report this week that saw net subscribers declining among all mobile providers, yet average revenue per user growing by 3%. All of which is good news for telecom investors, not necessarily good news for telecom customers.

One reason is that AT&T and Verizon have been signaling that they will try to lower subsidies for iPhones and (GOOG) Android phones in an effort to beef up margins. Carriers will pay, say, $400 of a new smartphone's $600 cost, but will lock subscribers into a two-year contract what will bring in nearly $2,000 for an $80 a month voice-and-data plan. High demand for iPhones have helped Apple (AAPL) win generous subsidy arrangements from carriers.

Recently, however, both Verizon and AT&T have indicated they want to spend less on subsidies, either by increasing the number of months before a subscriber can upgrade cheaply to a newer phone or simply by offering fewer subsidies in general. It's most likely that carriers will cut some subsidies and see if it leads to subscribers jumping to a competitor's plan, but even a little trimming of subsidy payments could help boost margins.

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The more effective way AT&T and Verizon are drawing more revenue from customers is in the elimination of unlimited-usage plans in favor of tiered pricing. As I've argued, these tiered-pricing plans are unlikely to appear terribly onerous to consumers as they're introduced, but instead will grow more costly as they use more high-bandwidth data like video chat and streaming movies on mobile devices.

Thanks to faster 4G LTE connections and the rising popularity of high-bandwidth apps, wireless data is already surging in demand. It used to be that big wireless carriers pushed unlimited data plans when in fact most subscribers used relatively little data, leaving them paying for expensive plans they didn't need.

Now that's changing. Many people who have LTE connections to their smartphones or iPads are discovering that they can be much faster than a wireless network set up a few years ago. So AT&T and Verizon are retooling their business models, introducing shared-data plans. Under these new breed of plans, wireless-data access can be linked to different devices, such as a smartphone and a tablet, while paying only one monthly fee.

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This week, Verizon introduced its Share Everything plan. On the face of it, the plan seems tailored to respond to subscriber demands for family data plans. But Verizon has carefully designed the plan on its own terms. It's not clear that the plans will save families money, and some calculations reckon that the new plans will be more expensive for some.

Verizon is also allowing unlimited voice calls and unlimited texts – features that would have been a real draw a few years ago, when people used mobile phones for voice calls and before the rise of free-texting apps like iMessage. But the shared-data plans seemed designed to push the family into more expensive usage tiers more quickly than ever.

And that's what shared data is all about: Kicking us into the tiered-pricing structure of data plans. Remember when Apple released the LTE iPads and people blew through their 2-gigabit monthly allotments in a few days? Imagine how long it will take for a family plan with as many as ten mobile devices to ascend into the more expensive tiers.

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As the biggest two mobile carriers in the country, AT&T and Verizon can't rely on new subscribers anymore. And they won't be able to rely on the transition from talk-and-text feature phones to smartphones for much longer: Half of all U.S. mobile subscribers already own smartphones. And many holdouts are waiting for better economic times before they upgrade.

So these companies are going to have to be creative about finding new ways to make money from each customer. And from the looks of things so far, they are getting very creative indeed.