When you join giffgaff, your voicemail is automatically switched on. You can access it by dialling 443. You can switch it off by dialling 1626 and reactivate it by dialling 1616.
To reset your voicemail PIN text RESET to 802443. You'll then receive a text with a temporary 5-digit PIN. Call the voicemail from your giffgaff phone to then enter a new PIN.