I'm not sure if Cingular has integrated all voicemail systems to have a
standard menu structure.  I know Sprint's voicemail system was exactly
the same for every Sprint phone, but up until now they've built their
network from scratch and could standardize it(as opposed to Cingular and
Verizon's "grow through acquisition" model, where a patchwork of network
and voicemail systems exist).  But this is how changing the greeting
works for me.  The procuedure for the phone you have may be different:

Step 1: Press and hold the 1 key (or voicemail key).  Alternately you
can save her a few minutes of wasted airtime by calling the cell phone
from a landline, and letting the call roll to voicemail, pressing the *
key to interrupt the greeting and log in.

Step 2: enter the passcode, if prompted.

Step 3: Press 4, then 3, then 3, then *

Step 4: follow the prompts to record and select a new greeting.

Step 5: Upon getting a job, save up to buy your own cell phone :)