Providence Equity Partners expects to complete the sale in April, having acquired Phones4U for £1.4bn in 2006.
Phones4U has emerged as a leading smartphone seller, with one in four of all smartphones in the UK sold through the company's 500 stores.
A BC Partners spokesman said there would not be any significant changes to the business operation, but more store openings were planned.
Andrew Newington, managing partner, BC Partners, said: "With its market-leading position, strong and trusted brand, multi-format strategy, and extensive store network, Phones 4U is ideally positioned to benefit from further strong growth in the smartphone market, as technological innovation and product evolution continues."
Phones4U reported sales growth of more than 20% to £900m in 2010.
The brand has used TV sponsorship extensively, sponsoring Sky's 'Must Be The Music', Channel 4's 'Hollyoaks' and, in January, it struck a six-figure deal to sponsor 'Harry Hill's TV Burp' in a deal brokered by Eden.