U.S. regulators on Friday asked Google Inc to explain how the Internet giant's Voice service blocks expensive calls to some rural areas, after lawmakers demanded an investigation.

The Federal Communications Commission asked Google to respond by October 28 with information about how Google Voice calls are routed, why it restricts calls to particular telephone numbers and how Google chooses the numbers.

Google must also identify how many users of Voice now exist and whether the company plans to offer Voice on other than an invitation-only basis, according to the letter sent by Sharon Gillett, chief of the FCC's wireline competition bureau.

Google has said that its Voice service is not a traditional phone call because it originates from a Web software tool, and has argued that it should not be regulated like telephone companies.

On Wednesday, a group of 20 U.S. House lawmakers, mostly from rural areas, urged the FCC to investigate Google's ability to block calls.

Google's Voice service has also triggered a dispute between the world's largest online search company and Apple Inc over why the voice application is not available on the iPhone.

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