Mobile phone firm EE has been fined £2.7million by Ofcom for overcharging tens of thousands of customers.
The communications regulator found that the company twice broke a "fundamental billing rule", resulting in nearly 40,000 customers being overcharged around £250,000.
An investigation by Ofcom accused EE of "carelessness or negligence", adding that while it did not set out to make money from its billing mistake, the company decided not to reimburse the majority of affected customers until the watchdog intervened.
Lindsey Fussell, Ofcom's consumer group director, said: "EE didn't take enough care to ensure that its customers were billed accurately.
"This ended up costing customers thousands of pounds, which is completely unacceptable.
"We monitor how phone companies bill their customers, and will not tolerate careless mistakes. Any company that breaks Ofcom's rules should expect similar consequences."
EE is owned by telecoms giant BT, which acquired the mobile operator last year in a £12.5billion deal.
EE customers who called the company's 150 customer services number while roaming in the EU were incorrectly charged as if they had called the US.
Ofcom said the mistake saw customers charged £1.20 per minute, instead of 19p per minute. At least 32,145 customers were overcharged around £245,700 in total.
It added that EE wrongly decided it could not identify the people it overcharged and was proposing to give their money to charity.
In another breach, despite making it free to call or text the 150 number from within the EU from November 18 2015, EE continued to bill 7,674 customers until January 11 last year.
These customers were overcharged a total of £2,203.33, although EE issued full refunds to those affected.
Vickie Sheriff, director of campaigns and communications at Which?, said: "If it wasn't for the regulator's intervention, thousands of customers would have been out of pocket due to EE's mistakes.
"Customers shouldn't have to wait for the regulator to step in to get an apology and their money back."
Proceeds of the fine will be passed to the Treasury.
While the majority of customers have been refunded, EE was unable to identify at least 6,905, who have been left around £60,000 out of pocket.
The company has made a donation of just under £62,000 to charity in lieu of the payments owed to these customers.
Ofcom is requiring EE to make further attempts to trace and refund every customer who was overcharged.
An EE spokesman said: "We accept these findings and apologise unreservedly to those customers affected by these technical billing issues between 2014 and 2015.
"We have put measures in place to prevent this from happening again, and have contacted the majority of customers to apologise and provide a full refund."