O2 has been fined £10.5m by telecoms regulator Ofcom for overcharging its customers.
For eight years, some customers who were leaving the network were double-charged some fees on their final bills, Ofcom said.
The error affected more than 140,000 pay-monthly subscribers between 2011 and 2019, totalling £2.4m.
But it has been unable to contact some of its former subscribers. Those refunds will be donated to charity.
Despite the refunds, Ofcom decided that O2 had broken important rules around providing customers with accurate bills and that it deserved a fine.
On top of the 140,000 who paid the extra amount, a further 110,000 people were overcharged a total of £38m – but did not end up paying the bill, and were not due a refund.
Ofcom said that O2 knew about the “issues with its billing processes” in 2011, but failed to fix them. It kept overcharging customers for a further eight years.
O2 said: “We are disappointed by this technical error and sincerely apologise to customers impacted.
“We identified the issue ourselves and notified our industry billing auditor. We have also taken proactive steps to refund all impacted customers for the extra charges they paid.”