MTN argue on the contract duration

I wanted to cancel my contract with MTN, from contract to prepaid and for 2 months I was told this it only expire end August 2016. I already started paying from May 2016, was supposed to lapse in May 2019. I have proof of the deduction, but MTN doesn't have my contract in hand and they argue. I will reverse the debit end of this month. I have never seen so much incompetence in trying to assist me with cancelling my contract!, but think this is done on purpose, why MTN, just cancel my contract as per what I signed! Now suddenly the contract I signed is at the shop and MTN doesn't have insight into this, but they argue!