There is no absolute safety. Make a contingency plan for your PBX.
In case an attacker successfully forced your PBX to fail, you should have a contingency plan for your PBX.
To get informed of the events that occur to your PBX, you need to configure Event Center on the PBX. Enable event notifications and add contacts to receive the notifications by email or phone number.
Prepare two PBXs that are in the same model and firmware version, and set up Hot Standby. When the primary server cannot work, you can quickly replace the failed server to the secondary server with the same configurations.
Schedule Auto Backup
Set auto backup on the PBX. If the PBX cannot work, you can reset the PBX, and restore the PBX configurations from the backup file.