HANDED OVER FOR NO REASON WHEN I PAID OFF MY CONTRACT

i was on a contract with vodacom which was suppose to end in October ,however i requested a settlement amount and i paid the amount and sent through the proof of payment ,my contract then went to prepaid ,yesterday i received a call from the debt collector saying i owe R300 i was shocked because i settle my account i even received an sms stating that ,Vodacom has never sent me any message regarding outstanding balance ???further more i was always on a debit order if this was outstanding why didn't it go of like a normal debit order now after 3months i get a call staing i owe money what **************** is this now??and i was contemplating taking out a new contract but with this dismal service i wont ,my contract was settled!!!