BT's mobile phone arm mmO2 has begun life as an independent company.
MmO2, which includes which includes Digifone in Ireland, has been listed separately on the London Stock Exchange and official trading is now under way.
Shares intially fell by 2p sterling to 78p in early morning trading, but later rose 5 per cent to 84p.
Investors traded heavily in the newcomer and, by mid-morning, more than 130 million shares had changed hands.
Formerly known as BT Wireless, mmO2 also owns mobile network operators in Holland and Ireland and runs internet portal Genie.
Mr John Tysoe, a telecoms analyst at West LB Panmure, says although trading in mmO2 has been heavy, investors are still unsure.
He says: "It is not clear cut what decision you should make about this. There are, crudely, two schools of thought.
"One says this company is never going to be anything else other than second league and is likely to struggle.
"The other says it does have some advantages, such as no debt, 17 million customers and, if all else fails, somebody might buy it."
As well as its London listing, mmO2 will start trading on Wall Street when the US markets open this afternoon.
Chairman David Varney says he is 'delighted' by the dual listing, adding: "Our management team is committed to delivering value for our shareholders."
PA