Apple Inc.’s voice recognition software Siri has led users of the iPhone 4S to use almost twice as much data as the handset’s predecessor, placing greater pressure on operators, network firm Arieso said.

“Voice is the ultimate human interface,” Arieso Chief Technology Officer Michael Flanagan said in an interview in London. “As you lower the barriers,” consumers will use their smartphones’ functions more often, he said. Arieso, based in Atlanta, advises clients such as Vodafone Group, Telefonica and Nokia Siemens Networks on how to manage wireless networks.

Apple brought in Siri, dubbed a virtual personal assistant, in its latest iPhone update in October. With a few spoken words, the artificial-intelligence feature helps iPhone users schedule appointments, write text messages and check the local weather.

The strain of data-intensive devices may place additional pressure on mobile operators as they build out faster networks. Verizon Wireless, the largest US carrier, said this week the number of iPhones it sold doubled to 4.2 million in the fourth quarter.

AT&T, which last year lost the exclusive US rights to the iPhone, has been criticized for dropped calls and network coverage in high-use areas such as New York and San Francisco.

A London-based Apple spokesman said he couldn’t immediately comment. The iPhone 4S, the latest update of Apple’s best-selling product, is Apple’s first major release since the death of Steve Jobs, who helped guide the company’s product design and marketing.

While the iPhone is the world’s most popular smartphone, Google ’s Android software is more widely used.

The iPhone 4S’ regular connections with Apple’s servers to synchronize applications including music lists may also contribute to the data load, Flanagan said.