No voice when call was answered

Person called up my Aunt, saying they were from Commonwealth Bank. Wanted her to "check" details. She got a notification that an attempt had been made on her Commonwealth Bank account to withdraw money. She is stressed to the Max, and changing banks asap!! Scary thing is they knew her bank balance.
Call says origin was Sunbury VIC 3429. The number is associated with Symbio Networks Pty Ltd

Caller: CBA

Received a call on the above phone number today from someone claiming to be Commonwealth Bank. The caller asked if I had tried to make an international transfer via my online bank account for $2,800. Provided recipients name and a transaction reference number. Then said they would transfer me to the Fraud department to cancel the transaction. I was transferred to their 'Fraud' department where I was asked to provide the disputed transaction reference and then I was asked to check my bank account for any unusual transactions - advised to not access my account via my laptop as it could be compromised and recommended to use the banking app as it was secured. The 'fraud' department caller asked me to check the transfer limits on each account - when I confirmed international transactions was $0, they said they had actioned this to prevent the $2800 from being taken. I was then asked if I my account was linked to any online gambling sites and cryptocurrency sites. I was provided with cancellation references for the international transactions. I was then guided through my banking app to check available balances, look for any international transfer fees or any low dollar transfers. When I confimred there weren't any, I was then asked to approve a transaction that they sent to my Commbank app. The transaction was a request to transfer $900 to "TapTap Send" - which he claimed would be rejected as they had deactivated international transfer functionality from my account. I was also told that after this transaction, to log on to my account and provide the transaction reference number - this was the confirmation that the fraudulent $2800 transfer would be terminated. I refused to complete the transfer request, the caller got agitated. I asked them to stay on the line while I called my bank to confirm this was a legitimate call. They said 'how would they know all this information if they weren't from the bank'. Caller said the bank would not be able to help me as the transaction code would be a one time reference. After hanging up, I got called 4 further times from similar numbers. In the meantime I go through to my bank who confirmed that no one had called me from CBA and that the call was a scam. Phone numbers have been provided to CBA Fraud Team.