********** transaction and poor service

On the 30th September,I reported a ********** transaction from my account.A purchase of a Cellphone worth R20 000 was done from MTN.I requested FNB to stop /reverse the transaction as it was an unauthorised transaction. They promised to make a follow up on the matter and update me Instead of helping me as their client,they are now telling me they can't help me .I'm so disappointed by their poor service ,please help .