Paid debt for 24months

I have been paying my account for the past 24 months i settled my debt last year October 2017 ,But I still get calls from the head office when I go to the branch they tell me that I don't owe them the balance is R0.00 .I asked them to give me a letter stating that I have finished paying .they told me to wait three to four days .since December I have been waiting for the letter up to date I haven't got the letter.all I ask is for you to intervene in this matter..Thank you