As much as 30 percent of homes in the United States and Britain could subscribe to Internet-based phone services in the next three years if major telecommunications companies offer services similar in price and quality to traditional phone connections... With nearly 200,000 subscribers, Vonage, which is based in Edison, N.J., is the market leader by a wide margin. But its share of the market has started to slip since AT&T introduced its Internet phone service, called CallVantage, in March. According to IDC, a market research firm, nearly 600,000 consumers in the United States are expected to subscribe to Internet calling services by the end of the year.