Internet telephony is expected to move from its early adopter phase and into the mainstream over the next two years, with the growing use of broadband by U.S. households being a key driver, a market researcher said Tuesday.

According to data collected from a variety of sources by EMarketer Inc., voice over Internet protocol, or VoIP, access lines in the United States could increase to as much as 26 million in 2008 from 9.9 million this year. Revenues from consumers using VoIP could rise to as high as $9.5 billion in 2008 from $1.92 billion this year.

Those numbers, however, are among the highest estimates for the industry, EMarketer said. Investment researcher Halpern Capital, for example, is far more conservative, estimating that U.S. VoIP services revenues would increase to $3.8 billion in 2008 from $370 million last year.

Conservative estimates for U.S. residential VoIP subscribers peg the number as low as 13.5 million in 2008 from 2.8 million this year, EMarketer said.

Nevertheless, the market research firm expects VoIP services to move into the mainstream over the next two years, because of the penetration of broadband. EMarketer estimates that more than 36 percent of U.S. households, or 42.3 million homes, would have high-speed Internet access by the end of the year.

"Despite the long road ahead, consumer and business migration to VoIP is on the verge of moving out of the early adoption phase and into the mainstream, and the next two years will mark the turning point when widespread VoIP use finally begins to take off," EMarketer analyst Steve Butler said in a statement.