Failure to cancel a ********** contract

I was contacted by Summer Mobile (who claimed to be phoning on behalf of MTN) in July as I was due an upgrade on my cell-phone contract. I agreed to an upgrade telephonically. However, when the cell-phone was delivered it became apparent that an upgrade was never done. Instead an additional line, contract and number was added to my current portfolio. I never agreed to this. I was subsequently advised by the MTN call-centre and agent Cindy from Summer Mobile to return the cell-phone to MTN, which I did on 09/08/2017 (Reference number: 256 354 761). I spoke to Sara (from Summer Mobile) who told me the contract would be cancelled within 7-10 days and then to Zalobia on 21/8/2017 who confirmed the contract was cancelled. However, I now see that MTN has billed me and the additional line is still linked to my profile. This is absolutely unacceptable - this contract was established **********ly, the phone has never even been used and it has been returned. I have been unable to contact MTN retentions telephonically as advised by the MTN call-centre (after more than 30 minutes wait on multiple occasions) and there has been no response to emails sent to MTN twice (reference # ********** 6). If MTN cannot resolve this - cancel the contract and reverse the charges immediately I will take further action.