O2 has overtaken BT to become the UK's biggest telecoms provider.
The mobile operator currently provides customers with 21 million mobile phone connections whereas BT has fallen behind with 19.4 million landlines.
O2 was originally owned by BT under the name BT Cellnet, but it spun the company out back in 2001.
Since being purchased by Spanish telecoms player Telefonica in 2006, O2 has gone from strength to strength, fuelled especially by its previous exclusivity deal with Apple to sell the ever popular iPhone which is now available on Orange, too.
The announcement comes in a week where BT reported profits falling by 45 per cent in the six months leading up to September this year.
A spokesperson from BT told IT PRO the company wasn't concerned as it had other avenues for its revenues.
"It is a nice neat figure that O2 has, but a mobile is a more of a personal decision whereas landlines are used by many people in the home or in business," he said.
"It is a weak argument as we are not driving our entire profits from calls and lines and we have never really played the numbers game in terms of mobiles and landlines."
We contacted O2 for comment but it had not responded to our request at the time of publication.
