DebiCheck not approved but Im still debited

I had notified FNB that they should not allow any further debits from Mr P Mobile as they neglected to cancel a termed contract from September 2021. I finally managed to cancel the contract in March 2022 only for Mr P Mobile to debit my account again this month May 2022. FNB However cannot verify me on my cellphone and thus cannot assist me to dispute the debit which i never authorised. They've sent me a msg that they cant get ahold of me , yet no missed calls from them? I have called 3 different days and I'm told the same story that they cannot verify me .. SO HOW CAN A 3RD PARTY GET INTO MY ACCOUNT AND I CANNOT??? It is beyond me, Is FNB working as a syndicate for companies to ***** and take your money without permission??? Can someone please answer me? FNB also does not allow for us to speak to a Team leader??? Is this service or day light *******????