Cancelled MTN line but still debits my account.

The worst service when it comes contract cancelations. My contract expired last year and I have tried a numerous occasions to cancel this *** [#***]. I havse sent all the required document. I was advised that this line will be migrated to pre-paid on the 03-04-2022. However I get an invoice for May. My email haven't been responded to. No one has called me. I don't know what else to do.