Contract cancelled but funds still withdrawn from my account

*** - I took out a sim only contract but I could not use the sim card because my phone does not have space for a second sim card. So on 26 March, the day I received the sim card, I phoned to cancel the contract. After at least 20 phone calls, between MTN and Mondo, and an SMS message to confirm the contract was cancelled from 31March, I received a reference number ***. I was also assured that the debit that went through would be refunded to me. Well, I have just received the latest statement, and guess what, instead of getting a refund, I have now again been debited for the cancelled contract!!!! So it seems that the contract has NOT been cancelled, despite telephonic and SMS confirmation. Please get this resolved immediately, this kind of service is totally unacceptable, as I am now out of pocket because of your inefficiencies!!!