Inexplicable demand for R 800 by cell C via a 'debit memo'.

My monthly cell C account had a DEBIT MEMO for R800. No explanation. I have contracts with Cell C Amelia Gaanakgomo at their Call centre said that she would get the accounts section to explain. She was very empathetic and polite but unfortunately my keypad would not record her excellent service. Just now I received two sms demanding that I pay the R800. I will have to speak to Law on Call as well about this inexplicable christmas present for cell C which deprives me of caring more for my family. I have been a loyal Cell C contract customer for over ten years. Please help.