Vodacom Cancellation

I cancelled my cellphone contract with vodacom but they keep sending invoices that I’m still owing. Now I got an email from the debt collector but they all fail to explain what the amount I’m owing for. I have proof that on the invoice they say I owe money for insurance but they debit the amount from my account every month. I have payed my vodacom account for 3 years and I never missed a month and it pains me that they don’t care about their customers. I don’t want to be blacklisted but I’m tired of paying Vodacom money that I don’t know