MTN ********ly deducts money off my account

iStore upgraded my cell phone contract on the MTN system but picked the wrong package. After completing the transaction on the system but before I'd signed the papers, the iStore person cancelled the upgrade on the system. I didn't receive the new phone and the contract was never concluded. The cancellation didn't work because MTN has now twice invoiced and deducted the higher amounts from my account as if the upgrade process had been completed. I've written to MTN a few times after being unable to get through to an operator on their 808 number. Nosithembele Nabo responded once by email to say they've requested the cancellation, but another month has gone by and they are seemingly unable to cancel the deal on their system and stop the incorrect invoices and deductions.