EE mobile
EE was handed a £2.7m fine by Ofcom for overcharging customers. Reuters

Mobile operator EE has been handed a £2.7m ($3.3m) fine by Ofcom for overcharging customers, after making what the telecoms regulator described as "fundamental billing mistakes".

The penalty is the result of a investigation into EE, the largest mobile network in the UK, which found the company overcharged approximately 40,000 customers for a combined total of £250,000.

EE customers who called the company's '150' customer services number while roaming within the European Union were incorrectly charged as if they had called the United States, meaning they paid £1.20 per minute, instead of the standard 19p rate per minute.

That translated in at least 32,145 customers being overcharged around £245,700 in total. Ofcom also found that, despite making it free to call or text the '150' number from within the EU from 18 November 2015, the operator continued to bill 7,674 customers up until 11 January 2016.

In total, the customers were overcharged £2,203.33, the regulator found, adding that the operator promptly atoned for its mistake in this particular instance and issued full refunds to those affected.

"EE didn't take enough care to ensure that its customers were billed accurately," said Lindsey Fussell, Ofcom's consumer group director.

"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."

Ofcom said EE's negligence contributed to these billing errors, and while it did not set out to make money from its billing mistake, the mobile operator declined to reimburse the majority of customers until the watchdog intervened.

EE was unable to identify at least 6,905 customers, who are owed over £60,000. While EE made a donation of just under £62,000 to charity in lieu of the payments owed to these customers, Ofcom has urged it to make further attempts to trace and refund every customer who was overcharged.

"EE wrongly decided it couldn't identify the people it overcharged and was proposing to give their money to charity, which would have left them out of pocket," Ofcom said in a statement. The watchdog added the fine must be paid within 20 days and will be passed on HM Treasury.

In an official statement, EE said: "We accept these findings and apologise unreservedly to those customers affected by these technical billing issues between 2014 & 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. For those customers that we could not identify, we donated the remaining excess fees to charitable causes in line with Ofcom's guidelines.

"Providing the best network experience and best customer service for EE customers in store, online and over the phone through our UK and Ireland-based centres are our top priorities. Following Ofcom's findings, we have made a number of additional improvements to our systems and policies to allow us to better support our customers in the rare occasion that billing issues do occur."