Won't Cancel My Contract

My contract was due to end on the 31st January 2016, I emailed MTN well in advance advising that I want my contract to end on that date and wish to no longer extend my contract. I was emailed advising that my cancellation request was received and I was asked if I would like to upgrade, etc. I rep**** saying please cancel my contract. Now, I receive a bill via email saying I owe money for February. I made it clear I wish to cancel and I refuse to pay for something I never used nor wanted as I was under the impression my contract was cancelled.