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The Associated Press

WASHINGTON – Google Inc. (Nasdaq: Goog) said late Wednesday that its free messaging and calling service, , blocks calls to fewer than 100 specific phone numbers likely to be adult chat lines and free conference call services.

The company made the disclosure in a letter to the Federal Communications Commission. The FCC opened an inquiry into call blocking by Google Voice earlier this month after AT&T Inc. complained that the service restricts calls to rural communities where local phone companies charge high connection fees.

So-called "common carrier" regulations prevent AT&T and other big phone companies from blocking those same calls. But Google, based in Mountain View, Calif., maintains that its Voice service should not be subject to common carrier laws because it is a free, Web-based software application, not a replacement for traditional phone service.

Google Voice lets users sign up for one number that can route incoming calls to cell, office or home phones. It also lets users place calls, including international calls, at low rates.

Google has already acknowledged that its Voice service restricts calls to phone numbers engaged in so-called "traffic pumping" schemes to collect access charges. Richard Whitt, Google’s Washington telecom and media counsel, said the company blocks these calls to "prevent these schemes from exploiting the free nature of Google Voice."

But in its letter to the FCC on Wednesday, the company said that after noticing "extremely high-cost calls to a concentrated number of destinations," it has been able to narrow down the number of phone numbers that it blocks to fewer than 100.

The tussle over Google Voice is part of a larger battle over a push at the FCC to develop "network neutrality" rules, which would prohibit phone and cable companies from favoring or discriminating against Internet traffic flowing over their broadband networks.

That effort has pitted Google and other Internet companies that support net neutrality against the big phone and cable companies, including AT&T, which want to be free of restrictions on what they can do with their networks.

For its part, AT&T argues that if the FCC imposes network neutrality rules on broadband providers, it should impose the same rules on Internet companies such as Google. To make its case, AT&T claims that Google Voice is flouting net neutrality principles by blocking certain calling traffic.

But Google maintains that the two issues are unrelated.