EE said it accepts a £2.7 million fine from Ofcom for overcharging tens of thousands of customers in 2014 and 2015.
The BT-owned company added that it “unreservedly” apologises to those affected.
An investigation by the UK regulator found that EE made two major billing mistakes, both of which centred on its 150 service number.
The first affected 32,145 customers who called the number while roaming within the EU, with EE incorrectly billing them as if they had called the US.
Instead of 19p per minute, they were charged £1.20 per minute.
Collectively, the customers were overcharged around £245,700.
EE had also decided not to reimburse the customers and instead give their money to charity until Ofcom intervened, having decided, wrongly according to the regulator, that it would be unable to identify these customers.
The second mistake saw EE continuing to bill 7,674 customers who called or texted the 150 number from within the EU until January 2016.
This was despite making this a free service since November 2015.
Collectively, these customers were overcharged £2,203.33 but have been refunded in full by EE.
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.”
An EE spokesperson responded: "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.
“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.”
Vodafone was fined £4.6 million by Ofcom last year over two breaches of consumer rules, which remains the industry record to date.
The company received the fine over a botched move to a new billing system that led to customer accounts not being credited with new top-ups.
It was also found that Vodafone customer complaints were mishandled.