SAN FRANCISCO  The European duo who created Skype and sold it to eBay for billions may have another trick up their sleeve: buying it back.

Niklas Zennstrom and Janus Friis, the founders of Skype, have approached several private equity firms and are pooling their own substantial resources to make a bid for the Internet calling service, say several people with knowledge of their plans.

The two men sold Skype to eBay in 2005 for $2.6 billion, and later received bonus payouts that increased the final price to $3.1 billion. Since then, Mr. Zennstrom, a native of Sweden, and Mr. Friis, of Denmark, have created the venture capital firm Atomico and backed the online video service Joost, both based in London.

Skype has more than 405 million registered users, up from 53 million when eBay bought it, and the service had $145 million in revenue in the fourth quarter of 2008. Calls are free between Skype users, and rates are a few pennies a minute for international calls to non-Skype users; the low cost has helped the company gain 8 percent of the world’s international calling minutes, according to TeleGeography, a market research firm.