I just bought a house that has this phone system and had to spend alot of time figuring it out since it didnt' come with manuals. Press the intercom button then enter "777" (on the keypad) then enter "0#" and then enter the password for the "main system" which is probably 1234 (if it hasn't been changed). THen press 1 to change the greeting. The system will give you the necessary prompts. If you don't have the passcode to get into the "main" system box, you may have a problem. Call ATT business phone section.