Vodacom wants to bill me on the settled account

I called Vodacom on the 20th of December 2014 to ask the premature cancellation quotation for my account, they quoted me R753.56 which i immediately pay the amount of R756.56 in vodacom shop in Woodmead and send back the signed cancellation form and the proof of payment, a week later i received an sms confirming that my account was settled and my cell phone number was converted to a prepaid. a Month later i received an invoice from Vodacom with the amount of R215 i ignored the invoice as i knew that the account in question was settled in December already, i received the same invoice again the following Month, i then called Vodacom account to ask how did i end up owing them that R215 and they said i must call the Cancellation department, i didn't call the Cancellation department because i didn't want to find myself in the situation like last time where i called almost every department and still don't have the answers, this morning i received an sms saying that my account was handed over to Debt Collection agency.<br> <br> I really don't understand how Vodacom is working, i am very disappointing with them