*****!! Unauthorized debit off my account. No Cell C contract!

Please explain to me. How is it legal for Cell C to deduct money off my cheque account without me even having a Cell C contract!!! This is *****!!! You have no authority to debit my account. Where did you even get my bank details from. This is the 3rd time I've had to arrange with my bank to reject the debit order. And yet, you still continue to deduct money off my account.