INSTALMENT QUERY

in January I received an invoice stating that I owe R301.00 while my instalment is R99.00 pm and did not skip a payment, I was communicating with them via email and i received an apology and that amount was never deducted, early February I received an invoice showing instalment to be paid end of the month is R99 and that R301 amount cleared. now my sim card is suspended and when I call they say I owe them R400, which includes penalties of more than my instalment