Cell C harassing my pensioner mother for a bill she already paid

My mother had a cell C account, which we transferred to a business account. At the end of December, my mother received a final bill from Cell C, which she paid on Jan 6th. In June, Cell C contacted my mother stating this amount was outstanding. On 5th June we sent the proof of payment to ********** My mum was contacted on a few other occasions, each time we sent proof of payment. On 8th October, we received an email from ********** confirming receipt of the proof of payment and acknowledging that the payment has been allocated. Last week, my mum receives a call from a debt collection agency, stating that she has been handed over. She called Cell C today. They acknowledged that they received the proof of payment but they allocated it to the business account, and cannot transfer it to her account which is showing as outstanding. I am trying to get hold of Kamal Singh, who apparently is the Head of Department, as nobody else seems to be able to assist me. I spoke to Christopher in the business department, Mahabi, and Simphiwe in legal, all of whom, were extremely unhelpful . the last person, Simphiwe was extremely rude and arrogant, telling me I need to write a letter to Cell C. There's nothing they can do. Can somebody please assist. Can somebody senior in either legal or Accounts from Cell C, please contact me.