The FCC action comes after AT&T, in a letter to the agency Sept. 25, complained that Google was pushing for net neutrality rules that would prohibit broadband providers from blocking or slowing Internet traffic while at the same time blocking some calls on its service to some phone numbers with high access charges.

"Numerous press reports indicate that Google is systematically blocking telephone calls from consumers that use Google Voice to call telephone numbers in certain rural communities," wrote Robert Quinn, AT&T's senior vice president for federal regulatory affairs. "By blocking these calls, Google is able to reduce its access expenses. Other providers, including those with which Google Voice competes, are banned from call blocking [by the FCC]."

Access charges are fees that telephone carriers exchange with each other for connecting phone calls to their networks. In some rural areas of the U.S., access charges are significantly higher than in other areas.

The FCC sent a letter to Google on Friday asking the company about its Voice service. The letter asks Google how many people have been invited to beta test Google Voice, how Google Voice calls are routed, and if Google blocks calls to some telephone carriers. The letter also asks if Google Voice competes with traditional telecom carriers.

The reasons Google Voice restricts calls to some local phone carriers is "simple," said Richard Whitt, Google's Washington, D.C., telecom and media counsel.