The number of consumer VoIP subscribers in the United States is estimated to reach 23.3 million by 2011, up from 11.8 million in the second quarter of 2007, according to TeleGeography's "US VoIP Research Service."

TeleGeography also projected that by 2011 VoIP penetration in Europe will be about twice as high as in the US.

"VoIP service in the US has emerged as a mainstream service that is causing traditional service providers some headaches," said Stephan Beckert, TeleGeography analyst, in a statement. "However, in Europe, VoIP could fundamentally change the structure of the fixed-line market."

European VoIP providers have better access to incumbents' local copper loops, which is driving the continent's VoIP growth. Aggressive competition and pricing and active participation by incumbent operators such as France Telecom, Deutsche Telekom and British Telecom are also drawing subscribers.

Incumbent service providers accounted for 26% of VoIP subscribers in Europe. By contrast, AT&T, Verizon and Qwest have not been especially competitive in the market.