Outstanding amount

My name is Andries Phofane, I had a cell phone contract with MTN back in 2013 ,I have paid up the contract and closed it after two years, but now when i tried purchasing a property, it appears as if I still owe them some money which is in arrears and affecting my credit score negatively, I went to the store and they enquired about my account but found no outstanding amount on my name. I requested a settlement letter but they have been taking me from pillar to post, telling me about turn around times and staff while I can not get any credit because of their fault.