Hi
If your unable to find anything in the file manager settings then you could look at a hard reset which takes the phone back to factory conditions. This will delete any saved data.
1st method
Go to menu or to notification
Click settings
Select Backup and reset -> Select Factory Reset -> Confirm
Select Reset Phone -> Erase everything -> Confirm
2nd method
Power off the phone
Press and hold Volume up (Red) + Power on (Yellow)
When options appear with Volume down (Blue) go to wipe data / factory reset and select it with Power on (Yellow)
Use Volume down (Blue) again and go to Yes and select it with Power on (Yellow)
Select Reboot system now with Power on (Yellow)
The phones will still be under their 24 month warranty so you could have Vodafone send them off for a diagnostic.
Current Phone >
Samsung Galaxy s²³ Ultra 512gb Phantom Black.