Failure to provide with paid up letter

I had a contract with Cell C which in my knowledge I have paid off. Suddenly I started to receive lots of phone calls from SSD demanding that I owe Cell C money. After I have tried to explain to them that I don't owe their client a cent I decided to settle the account regardless because I was annoyed by SSD calls. After settling the account I requested SSD to provide me with the paid up letter. SSD ignored my messages for weeks until I called them and one of their consultant told me that I need to wait for 21 days before I could get a paid up letter. Like the account is settled, they have proof of payment but they drag to write a simple paid up letter. It is only urgent and relevant when they call and demanded money from me but now they got what they wanted they ignore me and dismiss my request.