I need to rerecord the greeting message for my phone system.

According to the instruction manual, which can be found at: http://telephones.att.com/docMgt/public/art/175/1872_manual.pdf,

TO RECORD AN ANNOUNCEMENT:
1. Press A.
2. Choose the announcement location by
pressing s until the appropriate
mailbox is displayed.
3. Press R. Begin speaking after the beep
4. When you are finished, press S.
You will hear your new announcement.
5. Repeat Steps 1-4 to record another
announcement
— OR —
Press S.

TO REVIEW YOUR ANNOUNCEMENT:
1. Press A.
2. Press s to choose the announcement
you want to review (A, B, C, AO,
MBOX 3, MBOX 4).
3. Press P to hear the selected
announcement.

Or, if you're away from the office,
CHANGING ANNOUNCEMENTS:
You can re-record any of the announcements
currently in use by any
line or mailbox.
1. Contact the answering system.
2. After two beeps press a number key
(1-4) to select a line or mailbox, then
press 4*. The system beeps and begins
recording.
3. Speak into the telephone in a normal tone
of voice.
4. Press # when you are finished. The
system beeps once and plays the new
announcement.
To review your announcement, press a
number key (1-4) to select a line or mailbox.
Press 41 after the two beeps to play
your announcement.
NOTE: If announcement is being used by
both lines, re-recording the announcement
will change the announcement heard on
both lines.

Hope this helps!
tkhusa420