• by Mark Walsh  @markfwal, November 9, 2011

Despite the addition of a third carrier offering the iPhone, AT&T still claims the majority of subscribers on the latest version of Apple’s signature device.

More than half (56%) of iPhone 4S handsets in the U.S. are running on the AT&T network, according to new research from app analytics provider Localytics. The firm bases its estimate on the thousands of apps that run the company’s software to track user data on the Apple smartphone.

Verizon Wireless, which began carrying the iPhone earlier this year, powered service for 32% of iPhone 4s handsets, while Sprint had 12%. Sprint began only providing service with the new model.

Localytics surmised AT&T’s large base of long-term customers may give it an advantage in signing up customers for the latest iPhone. That’s because AT&T subscribers with older iPhone models may be at or near the end of their contracts.

“AT&T has been aggressively leveraging the iPhone 4S to sign these customers to new two-year contracts, and the data in this study suggests that tactic may be working,” according to the firm’s report. By contrast, Verizon iPhone users are still the first year of their wireless contracts, making the cost of upgrading higher.

With the prior-generation iPhone, the split between AT&T and Verizon was 60% to 40%, suggesting Sprint so far has taken more share from Verizon than AT&T. 

Sprint is counting on the iPhone to help compete with its two much bigger rivals in the smartphone game. Strong demand for the new device has benefited all three of the carriers to date. Apple said it sold a record 4 million iPhone 4S units in its opening weekend last month. Piper Jaffray analyst Gene Munster has forecast sales of 26 million iPhones in the fourth quarter, and 112.5 million for all of 2012. Both would be new records for Apple, according to AppleInsider.

But not every carrier necessarily covets the device. U.S. Cellular, the nation’s fifth-largest wireless operator, turned down the chance to offer the iPhone because Apple’s terms made the move too risky financially, according to CEO Mary Dillon. Sprint is obviously betting the gains of carrying the iPhone will outweigh the costs involved in subsidizing its price.