Cell C contract

I cancelled my contract with Cell C at the beginning of January due to bad service and never having signal. I paid the outstanding ± R2000 and was told that I will still be billed at the end of January for the normal contract rate and from there on that would be it. Come end of January nothing was debited. on the 28th February Cell C deducted the normal contract rate. I find this unacceptable as people have monthly budgets and they cannot debit when they feel like it. I feel that I should not be held liable to pay due to their incompetency to debit my account at the end of January and that Cell C should have contacted me arrange payment.