By David Lawsky

SAN FRANCISCO (Reuters) - U.S. mobile phone companies have begun to see substantial returns from delivering data and not just voice, fueled by greater openness on their networks, industry leaders said on Wednesday.

Resource Library:

But top executives of three of the nation's four largest mobile carriers also said they are still worried by consumer demands for unfettered freedom to use untested devices or software applications to connect to their networks.

"You are seeing the bulk of our opportunities really coming out of non-voice activities," Robert Dobson, chairman and president of T-Mobile's USA unit, said during a panel at a wireless industry trade show in San Francisco.

"Unfettered access would be a pretty bad experiment," Dobson said. "There needs to be some stewardship or control."

Industry trade group Cellular Telecommunications Industry Association (CTIA), the organizers of the conference, released new statistics showing that $14.8 billion of U.S. wireless revenue came from non-voice services in the first half of 2008. That's 20 percent of total U.S. wireless service revenue and a 40 percent increase over the first half of 2007, CTIA said.

The rapid growth in data services has been fueled by the success of Apple's iPhone with AT&T Inc and a race by rival carriers such as Verizon and Sprint to offer competing phones and data services with touchscreens and hot software.

Others expressed concern that if there was too much freedom interoperability would suffer.

At the same time, Dobson said networks would be "most productive with stewardship and control."