To change your voice mail greeting, pickup the handset, and dial 3566245, and when it asks for your pin, enter your voice mail pin followed by the # sign.

This will put you at the voice mail main menu.  To work with your greetings, press 3.  Next, press 2 to work with extended absence greeting.  You will press 1 to record the extended absence greeting.  Follow the prompts.  Once you have the greeting recorded to your satisfaction, hit 3 to exit.

Now, you will need to turn on the extended absence greeting.  Hang up the phone, and dial 3566245 again and your pin again.  Hit 3 to work with extended absence greeting.  When she says "your extended absence greeting is turned off."  Follow the prompts to turn it on.