If this is for your mobile, dial your own number, and when you start hearing your current greeting, enter your 4 digit PIN and press #. This gives you remote access to your voicemail to check messages etc.
From there you can select the option for personal settings etc to change your greeting.
I work for Vodafone, supporting our mobile and fixed customers, and volunteer my time on the community.
PM = Private Message. Click on my name to send me a message.