Bad service

Good morning

I informed cell c two months before my contract expire that i'm going to cancel my contract. I followed up the following month and confirm that i want them to end my contract as soon as my contract expire. Was assured all is fine and contract is cancelled. I received my bill from Cell C and guess what according to them the cancellation was captured late and therefore i'm billed an extra month. My question is if i gave more then enough notice to cancel my contract am i being held accountable for your incompetency? Why must i pay because a consultaant did not do his or her job?