Mobile network O2 has been sending some of its former customers refunds, 15 years after the end of their contracts.
Several people posted on Twitter that they had received a letter and cheque from parent company Telefonica stating: "You've got a refund."
Some were suspicious that the letter was sent by cyber-fraudsters.
But O2 confirmed that it had sent cheques to a group of people who were over-charged more than 10 years ago - with interest.
"We identified a billing issue which meant some customers were charged twice on their final bill before leaving O2. We have been contacting those affected to apologise and send their refund," the company said in a statement.
One customer said they had been offered a refund of £127, while another was sent a cheque for about £2.
In the attached letter, Telefonica said the refund amounts included 4% interest.
"We have a responsibility to do all we can to reimburse any customer for what's rightfully theirs, even if they've since left the network and it's a low-value amount," the company said.
O2 said it had identified a group customers it had over-charged at the beginning of the year, but had delayed sending out cheques until coronavirus lockdown measures had been eased. It told the BBC it was not a common issue.
The company also said it had made efforts to trace former customers' latest addresses, so that it could issue the 15-year-old refunds.