Cancelled Contract but still got billed

In March I cancelled my contract as I no longer use the data and the contract period has ended. I was obviously already invoiced for March, which is fine, and was paid by debit order.

Now in April, I got invoiced again for the same amount as before, so I had to call the cancellations department again. I then got an email saying my contract was converted to prepaid (which is fine), however, the effective date is April 30. The email also states that any outstanding costs will be billed NEXT month. What outstanding costs??

While still on the call, I asked the agent what about the invoice that I'm being billed for in April that will go off by debit order - he said I'll be credited.

The joke is, as of 1 April - I have been unable to use the simcard. So I was billed for something that I cannot even use. Essentially that can lead to theft.

Reverse the bill, with no charge for April. I would like a new invoice with R0 charged to confirm that I will not be charged incorrectly.