Unauthorised debit order

An amount of 99.99 with the reference MZANSI4U has been debited from my FNB account. I have not authorized this debit order. I am unable to dispute or reverse this debit order from my FNB portal because they attempted to collect but they we not enough funds on my account. I am afraid they will try to collect again next month. I have contacted them to cancel the debit order on ********** 430 but they dropped the phone on me. my ref number is MBSMZ **********