Annoying contract cancellation process

I'm so disappointed at how vodacom is handling my contract cancellation. It's such a strenuous and frustrating process. Every time I call to cancel my contract, they say I owe, whilst I have already paid the bill. The amount they say I owe is a bill for the next month. Like what the hell! I'm almost on the third week trying to cancel their contract and still, I a being told I owe whereas my statement clearly shows that they have debited their money.