The O2 mobile network sent some of the refunds to its former customers 15 years after their contracts ended.
Several people posted on Twitter that they received a letter and a check from the parent company Telefonica stating "You got a refund."
Some suspected that the letter was being sent by cyber-scammers.
But O2 confirmed that it had sent checks to a group of people who had been overburdened more than 10 years ago - with interest.
"We identified a billing issue that resulted in two customers being charged to the final account before leaving O2. We contacted interested people to apologize and send the refund," the company said in a statement.
One customer said they received a £ 127 refund, while another sent a check for around £ 2.
In the attached letter, Telefonica stated that the refund amounts included a 4% interest.
"We have a responsibility to do everything we can to repay any customer for what is legitimately his, even if they have since left the network and it is a low value amount," said the company.
O2 said it identified a group of customers earlier in the year, but delayed sending checks until coronavirus blocking measures were relaxed. He told the BBC it wasn't a common problem.
The company also claimed to have made efforts to track down the latest addresses of former customers, so that 15-year refunds could be issued.
