Stop debating my account,I no longer have a contract with you

I opened a contract with mtn last year November,they told me that its ending this year November.I have been paying them each and every month,I can even print out statements from my bank to prove this.The contract was supposed to end last month in November,they are continuing to debit my account but my phone is changed from contract to prepaid.I first went to the branch,they told me that they cannot do anything to assist me I must call 135 and ask to be transferred to contracts,when I called 135..I firstly got bad service n my line kept going off,I domt understand whether their service champions were hanging up or what,when my call finally went through they told me that I have to pay for December Atwell coz they changed my contract to prepaid on the 15th of December,now they are sending me massages saying that they are going to cut my line if I don't pay them and send me to their lawyers..I am really not happy about this coz I even have their massages from last month saying that my contract has been canceled n its now prepaid and I can prove all my payments n I made it clear to them that I don't want to upgrade.please help