That "system" greeting cannot be changed - so what you need to do is specify in your Day/Night autoattendant greeting what you want callers to do. What I record is:
"To Dial-by-Name, press *2, and, when prompted, enter up to the 1st 4 letters of your party's first (or last) name".
If it's a newer messaging you can say :"enter your party's first or last name (newer messagings allow encoding up to 10 letters for the Directory listing.
Tom Daugirdas,
President
STCG, Inc.
stcg.com