MTN keeps billing even though Cancellation Request sent through

I have a contract with MTN that expired on the 16th of February 2018. In January, before 20 business days, I had phoned MTN in regards to cancellation of my contract. I was told that if I wish to keep my number and port it onto prepaid then I must call on the day of my cancellation so that they can cancel my contract and I can keep my number on prepaid. I called on the 16th of February 2016, but the signal was ********** and the lady at the call center was not able to hear me properly. She advised me to email their cancellation department, requesting for the cancellation of the contract in addition to porting my number to prepaid. This email was promptly sent with a copy of my ID and all my particulars on the 16th of February 2018. However I was still billed in March. Again I sent an email on the 4th of April 2018, yet again the contract has not been cancelled. I have sent an email again on the 16th of April, without any reply. This is truly shocking as MTN keeps on billing me, yet I have sent countless emails to cancel my contract.