(Business 2.0) – Before most of his friends were out of college, Jeffrey Citron earned his first $1 million as a trader. At 25, he launched Island ECN, an electronic trading system for brokers that later sold for $500 million. By his 27th birthday, he'd turned his fledgling online stock brokerage, Datek, into the nation's fourth-largest. Ameritrade snapped it up in 2003 for $1.3 billion.

Now 34, Citron—whose personal fortune is estimated at more than $340 million—looks poised to strike it rich yet again. In 2001 he was among the first to dive into the uncharted waters of the voice-over-Internet phone business, putting up $70 million of his own money. Today his New Jersey-based company, Vonage, sits atop a sizzling $3 billion-per-year market.

At the moment, Vonage has more customers—about 350,000—than any other VOIP player. But given that his rivals include telecom and cable giants like AT&T, Comcast, and SBC, that lead would look tenuous without Citron's nimble footwork during the past 12 months. To fund expansion, Citron took his pitch to top-tier venture firms like 3i and New Enterprise Associates, which handed him a combined $140 million, making Vonage the largest private equity investment of 2004. Citron also lobbied heavily for and won a ruling from the Federal Communications Commission that keeps VOIP services exempt from state regulations—laws that might have buried his company.

With more than 400 smaller VOIP outfits chasing Vonage, and with the telecoms circling, Vonage's venture windfall is just as much a bet on Citron as it is on his company. "He's the rare guy with a tremendous command of both the minute details and the overall strategy," says J. Sanford Miller, managing director of 3i. "You don't often see that in a CEO." — OM MALIK