Cell c refusit to issues the payed up letter

Greetings, I took a cell phone contract with cell c, lost my job and got black listed, 3 years later got the job, called cellc, payed the remaining balance. My problem now is that they don't want to give me my payed up letter, my battle with cell c has been going on for more than 18 months. Apparently cell c handed my account to NDS the collection company but the company (NDS) says my ID number does not appear on their system. Now I am stuck I don't know what to do, but what I can day is that cell c is not good with customers when they are in trouble.