Cancellation Letter sent and contract not cancelled

On 14 November 2017 I sent a signed letter to MTN that stated that I wish to cancel one of my contracts when it reaches it's 24 months (which was on 1 January 2018). I also stated very clearly in this letter that i would like convert the line to pre-paid. I have just noticed that in January and then again now in February, I have been debited for this line. Phoning the call centre to rectify this matter leads to pointless excuses that evade the facts. I sent a cancellation letter, more than 30 days notice given, I expect this to be cancelled and I expect to be refunded in full for this line for the unauthorized debits that have been taken for this line. This is extremely frustrating and disappointing from MTN who I have been with for almost 7 years!