AFAIK O2 customer service can reset this for you by setting up the maibox again.
Maybe you can try to record a new message and then delete it which may return it to default?
To change your personal greeting
• Call 901 from your mobile
• You will hear the main menu options - select option 'three'
• Another set of options - select 'two' to record a full greeting, or 'three' to record a new nametag
• Follow the prompts to complete the change
To change your PIN
• Call 901 from your mobile
• You will hear the main menu options - select option four
• Follow the prompts to complete the change
To retrieve your voicemail messages from a landline
• Switch off your mobile phone
• Dial your mobile number from a landline
• Interrupt the voicemail greeting by pressing the * (star) key
• Enter the four digit pin code (default is 8705)
• Follow the prompts to listen to your messages