Though AT&T is allowing subscribers to download and use the iPhone Skype application, it says the software is only for use on Wi-Fi networks and not on the much more widely available 3G system. (Deutsche Telekom went further, banning use of the application altogether for its subscribers.) Though AT&T iPhone users are required to sign up for a combination voice and data plan, carriers' general fear is that subscribers could use Skype instead of paying for minutes. Asked about its policy, AT&T has said it expects its vendors, such as Apple, not to promote the services of a rival.

Carriers that sell BlackBerrys are likely to raise objections to the deal between Skype and Research in Motion, too, said IDC analyst Will Stofega. The open-Internet group Free Press last week asked the U.S. Federal Communications Commission to investigate moves by AT&T and others to apparently block the use of VoIP.

But 4G networks may transform the competition between carriers and third-party, "over-the-top" VoIP companies, because any voice service carried over these networks will take the form of IP packets.

Asked what it could do to counter third-party providers once it starts delivering VoIP itself, Verizon Wireless gave hints as to how it plans to approach data services in general.

"We have moved away from unlimited data plans," Verizon Wireless President and CEO Lowell McAdam said at a question-and-answer session at CTIA. "The excitement of an over-the-top application like (Skype) in an unlimited environment means one thing to a customer. In an environment where you're paying for every byte, that means something totally different." McAdam said.