Cell C is bad especially on contracts

I had a contract with cell c 2019, 2020 due to covid I couldn't pay my installment they took my account to debt collectors the Blakes. 2021 I paid all the amount I was owing by November till today they tell me I'm owing 1400. I sent them proof of payments even today nothing has happened they will tell me that they will attend the matter nothing has happened. I emailed them more than 3 times, send texts on social media no help.