Paidup letter

I had a contract phone from 2020 till 2023 with them and I have finished paying the last year December 2023. Then when I asked for a paidup letter they told me that they must Refund me R600 that I have overpaid them. I am still waiting for the refund and paidup letter. When I check my itc I found that blue label wrote that I own them R1 236, I have paid them R 1260 on the 6th of December 2023.