Cell C bulling tactics

My contract with Cell C ended in February 2023. To my knowledge I paid all instalments due to them. Cell C decided to delete my number for no reason and upon enquiring I was told I owe the last installment. I made an arrangement to pay them end of September but I couldn't for financial reasons. Then on Nov 13th I paying the whole amount and sent them proof of payment. I have been receiving call and email for non payment even though I sent proof to them and their collection department. Today a different company called and claim I owe even a bigger amount. These people are bulling me. Now there's also an adverse warning against me on Experian.