The Federal Communications Commission has opened an investigation into Google for preventing users of its free voice service from making calls to some rural phone numbers.

Sharon Gillett, chief of the FCC's Wireline Competition Bureau, sent a letter Friday to Google telecom counsel Richard Whitt asking a host of questions about Google Voice. Among others, Gillett asked why the company blocks some calls, how Google informs users about restrictions to rural numbers, and how Google believes its service should be classified for regulatory purposes.

Gillett said the agency is conducting the probe in light of its rules against call-blocking by carriers, concerns about "access stimulation" -- when rural telcos take advantage of the ability to charge national carriers high rates by partnering with teleconference centers and adult chat lines -- and interest in ensuring open broadband networks.

Whitt said in a blog post Friday that Google Voice blocks some calls to rural exchanges because it could not afford to offer the service for free otherwise. "The reason we restrict calls to certain local phone carriers' numbers is simple. Not only do they charge exorbitant termination rates for calls, but they also partner with adult sex chat lines and 'free' conference calling centers to drive high volumes of traffic," Whitt wrote. "Google Voice is a free application and we want to keep it that way for all our users -- which we could not afford to do if we paid these ludicrously high charges."

Some rural telcos charge other carriers very high rates -- up to 100 times more than large local phone companies -- to connect calls coming from long-distance or wireless numbers.

The FCC's move comes just two days after 20 lawmakers asked the agency to investigate Google Voice. AT&T also recently complained to the FCC that Google was violating agency rules against call blocking and neutrality principles by refusing to put through all telephone calls.

Google says that it's not a traditional telecom, and therefore, rules against blocking don't apply. The company also says that net neutrality rules only apply to Internet service providers, and not companies that create Web-based software apps.

But the FCC seemed to indicate that it's at least considering whether Google's call blocking is inconsistent with the agency's neutrality principles, given its statement that one of the reasons for the probe is to ensure open broadband networks.

Still, the letter to Google leaves plenty of room for interpretation. Chris Riley, policy counsel with advocacy group Free Press, said he does not believe the FCC's letter shows that it currently views Google's call blocking as a violation of neutrality principles. "They're trying to show that they're not going to rule out anything until they get all the data," he said.

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