Cancelled contract but still being invoiced

I cancelled my contract with Telkom Mobile in December 2015 as the contract had come to an end. I went to the Eastgate branch and gave them 1 months notice and was given Case number ***3 I was assured by the lady that assisted me that my number would be changed to prepaid and that I would receive no more bills. January 2016 they never sent me any more bills or took off any debit orders. On 28 January 2016 I received a bill in my email for almost R500 I called the call centre and they told me to my horror that my contract is still active I demanded to know why and was told to go back to Eastgate and cancel again and I was told that I would not get billed R500 only R70 still don't know for what as I never got an invoice. I went to the Eastgate branch on Friday 29 January filled out another form and was given Case number ***1 and was told someone would call me from the call centre. Now I have received another bill for R513.50 for the month of February 2015 when my original contract price was R300 per month. I tried to call the call centre today twice and was on hold for over 10 minutes each time and could not get through this is disgusting service please call me to sort out.