vodacom dispute

I ended my contract in February of this year they changed my sim card to prepaid them 4 months later I get a sms saying I owe them money.according to vodacom my account had to be paid up before they change my simcard to prepaid.so I cnt understand why I should pay them I asked for a details statement asking how did they get the amount I'm still waiting and NOW they handed me over.I'm so damn fed up