I had a cellphone co...

I had a cellphone contract with Cell C from November 2020 and called the cellphone company requesting a cancelation and the amount due to them, they provided me with all the details including their banking details and I paid off the account July 2021(R10 000) and the cellphone company said they never received the payment even though there is proof of payment to prove that I did make the payment and they are refusing to give me a paid up letter and now I have a bad credit record, because it still shows that I still owe the cellphone company money that was paid off