A "small" number of EE customers will share a refund of roughly £1m after they were wrongly charged VAT.
Customers who went outside of the EU and used internet data between October 2012 and October 2014 are affected - about 0.5% of EE's total customers.
The company told the BBC the money "was never EE's" and that the overcharge, blamed on a system-configuration error, went directly to Revenue & Customs.
Refunds will range from about £2 to £80 per customer.
"Due to a configuration error in our billing system, made following a system change, a small number of customers were wrongly charged VAT on the Data Roaming bundle outside of Europe," spokesman David Nieberg said.
"This was a mistake, and we are now refunding these charges and contacting affected customers to apologise for the error."
He added: "We've claimed that money back from HMRC, and then it goes back to the customers."
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EE has about 28 million customers, making it the largest mobile provider in the UK.
Customers affected by the error are being contacted via text message and told the amount they are owed.
EE would not be offering a cash refund, it said, but would instead give customers credit on their accounts.
Former customers who believe they may be eligible have been told to contact customer services.
The mistake had been discovered after a customer complained, the company said.
In December it was announced that BT was in talks with EE over a possible £12.5bn takeover.
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