O2 customers are being sent refunds for charges they incurred back in 2013 after leaving the network - some as small as 18 PENCE.
The company said customers are being sent rebates for incorrect exit fees that go as far back as six years.
It said it's been writing to customers who switched away from it, explaining that they were overcharged on early termination penalties.
O2 was unable to confirm the number of customers affected, but said the refunds are small.
A number of former customers have posted on social media about their refunds, with some including images of the letters they've received from O2.
The letters include a cheque for the refund, plus 4% interest.
Have you been sent a refund from O2? Get in touch: emma.munbodh@mirror.co.uk
Early termination charges are typically charged when a customer leaves their contract before their minimum term is over.
O2 generally works out these fees by adding together all agreed monthly airtime tariff charges for the remainder of your minimum term, factoring in any regular recurring discounts you’re entitled to and then deducting VAT.
On twitter, one customer wrote: "Seems I was overcharged by O2 in 2013 so they kindly sent me a refund yesterday... I have a cheque for 18p!!"
Another added: "Appreciate the honesty and of course for your financial books too @O2 giving me £0.45 cheque for an early termination issue back in 2016."
O2 said it is proactively contacting customers who have been overcharged and sending them a cheque, so you shouldn't need to do anything. However, if you think you were overcharged, you can get in touch with the network, here.
An O2 spokesperson said: “We identified that some customers who left O2 from 2013 onwards had paid an incorrect early termination charge. Any customer affected has been reimbursed.”
