The U.S. voice-over-IP market continues to grow, surging from 1.9 million subscribers after the first quarter of this year to 2.7 million subscribers after the second quarter, according to the latest research from TeleGeography. While that quarterly growth rate of 40% is impressive, the even more astounding stat is the 600% increase in subscribers that the market has experienced since the second quarter of 2004. Of course, with a surge in subscriber sign-ups also comes a surge in revenue. TeleGeography's research, published by PriMetrica, points to an increase in U.S. VoIP market revenue from $151 million in the first quarter this year to $220 million in the second quarter. The strong year for VoIP in the U.S. so far has prompted TeleGeography to adjust its projections for the end of this year. The research firm had anticipated 4 million VoIP subscribers in the U.S. by the end of this year, but stated at the end of the second quarter update that the number should end up closer to 4.2 million.