BOULDER, Colo. -- Leading Voice over IP service provider Vonage Holdings has complained to the Federal Communications Commission that competitors are blocking the use of its service, according to FCC chairman Michael Powell and others close to the company.

"We're very actively on this case and we are taking it pretty seriously," said Powell, during an interview Monday here at the Silicon Flatirons conference. In a speech at the conference Sunday, Stanford law professor Larry Lessig said that Vonage has been telling the FCC that other service providers are hampering Vonage's VoIP service by "blocking" it from reaching certain SIP addresses for end-user devices.

According to Powell, his understanding is that the blocking is not coming from major service providers, but from rural Local Exchange Carriers (LECs). Brooke Schulz, Vonage's senior vice president for corporate communications, said Monday that the company would not comment on the report.

(UPDATE: The company has now confirmed the report; click here for the latest story.)

Reports of other providers using networking techniques to block competitors' VoIP services have surfaced before, but none have involved Vonage or major U.S. service providers.

Since new sophisticated network-management tools allow service providers to determine the types of traffic flowing across their networks, the possibility exists that certain types of traffic -- such as Vonage's VoIP services -- could be "blocked" or otherwise degraded.

And since there is no current law or regulation prohibiting such techniques, it's unclear what Vonage's complaints to the FCC might accomplish. But Powell said the FCC might indeed have some enforcement options, specifically if carriers are found to be violating anti-competitive statutes.

Lessig's claims gave other speakers at Sunday's program more fuel for the telecom regulation reform debate, with most decrying any such packet-based discrimination.

"If Vonage is toast... because the ports are blocked, that's not good," said Phil Weiser, associate professor of law and telecommunications at the University of Colorado, the host of the Silicon Flatirons Telecommunications Program.

MCI executive and Internet co-founder Vint Cerf agreed, saying it was bad for everyone if service providers suddenly started discriminating against traffic types by competitive parameters.

"The presumption [of the Internet] is that you're fully connected," Cerf said. Any attempts to block certain application types or types of content, he said, "will destroy the utility of the Net."

Qwest CEO Richard Notebaert, who also spoke at the conference Monday, said his company has a commercial contract with Vonage. Carrying more application traffic, Notebaert said, was an economic plus for Qwest.

"I want to run a network utilized as fully as possible," Notebaert said. "I want to sell wholesale [access] to everyone I can."