O2 was fined £10.5m on Friday by telecoms regulator Ofcom for overcharging more than 140,000 customers between 2011 and 2019.
Over the eight year period, 250,000 customers who had told the network they were leaving were double charged for some fees. Around 140,000 customers paid those fees, amounting to a total of £2.4m.
O2 has refunded many customers, adding an extra 4pc. But the company has been unable to find others, instead donating their refunds to charity.
Even despite the refunds, Ofcom said O2 had broken important rules about bills being accurate.
Gaucho Rasmussen, Ofcom’s Enforcement Director, said: “Mobile customers trust their provider to bill them correctly and fix any errors as quickly as possible. But these billing issues continued for a number of years without sufficient action from O2, and thousands of customers were overcharged as a result.”
He added: “O2 has refunded the customers who were affected, and we are satisfied the company has taken action to prevent this happening again.”
O2 blamed a technical error and said they “sincerely apologise to customers impacted”.