Billing terminated service

I took a contract with Telkom for mobile home phone/wireless. In march 2015 my account was in arrears, I went to the store at Long beach mall to settle my debt. After doing so, I asked the consultant to terminate my contract, he agreed. Made a copy of my ID and did the necessary. In April I got a bill, I went back to Long beach mall and they told me to disregard it. Now I've been receiving calls and SMS from a debt collector and when I call Telkom they always tell me they are investigating the matter buy I never get feedback. The debt collector even threatened to list me at credit bureau. How does Telkom bill an account that is closed.