Unnecessary charges

Am so frustrated about MTN acc number BA*** cellphone number ***. I was having a cell contract with them and it was full paid after a certain period they called me and demand the money while the account was paid in full. I tried to explain and asked questions regarding the amount they are demanding and they are failing to provide me with full details and the supportive documents they are referring to. Currently am listed am no longer having access for any credit, am humbly request the MTN to review my account and clear me as a matter of urgently.