Upgrade Dispute

I have been dealing with this matter for months and have yet to receive any response or update on this matter. Last year Nov/Dec i was called and informed that i was due for an upgrade on my contract, i was then informed about the type upgrade and the handset i would receive should i proceed with the upgrade. I then informed the sales women that i was currently not interested in upgrading as my contract was coming to an end. At the end of my contract when i tried to close the account after final payment was taken by Cell C i was told i couldn't as an upgrade was activated on my account. I requested proof as to who signed off on this upgrade and who signed off for the handset, this info was never given to me, eventually i was able to close the account late February as after i did a follow up on the matter and was told the matter was resolved. but it seems that there is no communication between these departments as i was then informed that i owe them R549.33 for the upgrade. What i have yet to understand is how i am liable to pay this amount when the upgrade was *******ly done on MY account? I had to call again this morning requesting an update on this matter.