O2 continued to take £50 every month out of Peter Young's bank account after he claims he declined to renew his contract in June 2006.
"I was horrified at this and told O2 that I had cancelled that phone and it had never been used since June 2006," Mr Young, of Blyth, told Chronicle Live.
"I wrote to O2 complaints and they refunded half of the money as a goodwill gesture.
"But I am still over £2000 short."
The mobile phone operator confirmed Mr Young had taken out a 12-month contract in 2005 but said they had no record that it had been cancelled.
O2 says it has no record that Mr Young cancelled his contract
"We investigated Mr Young's account when he contacted us in May 2016," O2 said.
"We had no record to cancel the service he'd taken out in 2005.
"As a gesture of goodwill we agreed to part refund the monies paid.
"We would encourage everyone to check their bank statements regularly and, if they identify a discrepancy, to contact the organisation concerned."
O2's argument is that they continued to provide a service that Mr Young could have used.
They said they had continued to send monthly bills to his registered home address and then to the registered email address when bills went paperless in 2010.
"We have also written to Mr Young this week to explain that the amount we have credited back last year is our final position and advised that he may wish to contact the Ombudsman," they said.
Direct debits can be very helpful with paying bills, saving time and effort and getting rid of any worries about being fined for a late payment.
Some gas and electricity providers give you a discount for paying by direct debit and the bank will pay incorrect payments back to you.
But there are disadvantages.
A spokesperson from the Money Advice Service said: "You need to stay in control. Keep track of your direct debits and make sure there is enough money to cover the payments.
"Try setting yourself a reminder to check. This is easy to do, especially if you have online access to your account.
"Watch out for refused payments. If you don't have enough money in your account to cover a direct debit your bank can refuse to make the payment and might charge you - typically £5 to £25.
"Even if they do make the payment you might go into the red without noticing – which means you'll have to pay overdraft charges."