Harassment about an account I have no knowledge of and debit order

Good day

This company arrases me, telling me that they are phoning on behalf of CellC and my account I have no knowledge of and they debit my account every month with different amounts. I have stoped the debit with my bank numerous times and they refunded me. They debit my account without my content. I have also requested them to provide me with a statement and til Today no email.statement received. How can I stop and resolve this *****?!