"If you look at the consumer rights [spelled out] in the FCC's Internet Policy Statement, there is a chance that they might be violation by this practice," said Chris Riley, the policy counsel at Free Press, a Washington-based media reform group.

The group's letter cited the release Tuesday of Skype for Apple's iPhone as an example for the need to clarify the rules. Skype, which its maker said yesterday had been downloaded more than 1 million times since Tuesday, allows VoIP calls only via a wireless connection on the iPhone. However, iPhone users can't make VoIP calls on the carrier's own data network.

In the U.S., AT&T is the exclusive carrier for the iPhone.

AT&T claimed that it doesn't block VoIP traffic on its 3G network. "We do not prohibit VoIP," said company spokesman Mark Siegel. "But we expect our vendors not to facilitate the services of our competitors. We shouldn't have the obligation to promote our competitors."

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