The combined value of four private equity groups' share stakes in Vonage, the US internet telephone company, fell by about $277m (€216m) in its first two days of trading.

The company raised $531m last week through Citigroup, Deutsche Bank and UBS in an initial public offering that has been one of the worst performing in the US this year. Shares were issued at $17, lost 13% on their first day and then closed down a further 12% the next day.