As of September 2009, O2 had 21 million mobile customers in the UK - up 5.7 per cent year on year.
In a blog post, O2 claimed that "that's enough to fill 233 Wembley Stadiums!".
Between them, these 21 million mobile customers spent 39,680,000,000 minutes on the phone.
"If one person made all those calls it'd take them more than 75,000 years," commented O2.
O2 Broadband now has 527,126 customers.
Matthew Key, Chairman & Chief Executive of Telefónica Europe, commented: "Telefónica O2 UK maintained its market leadership, yet again outperforming its competitors in a more competitive environment.
"We added 292,000 net mobile customers in the quarter, bringing the mobile net adds for the first nine months of 2009 to 686,000, with an outstanding 22.5 percent year-on-year increase in the contract segment, which made up 44.5 percent of the total customer base at the end of September 2009.
"O2's ongoing success in the UK is clearly down to our strategy of putting the customer first - our levels of churn are the lowest in the market and the popularity of our propositions such as "Simplicity" and our wide range of high-end devices continues to pay dividends.
"The use of mobile internet is also driving growth as increasing smartphone numbers are impacting on data ARPU figures - which rose 5.1 per cent in the third quarter."