THIS PERTAINS TO THE PHONE
Unplug your phone and with the # key pressed plug the phone back in. Depending on your dexterity you may need a second person. You should see LCD testing displayed. All LCD positions should show a dark block. (if this fails, you may have a defective phone) You can unplug and re-plug the phone if you don't want to cycle through all the tests. The # key will cycle to the next test which will be the the button LED's. (if it was just a fluke with the phone this may fix it)
THIS PERTAINS TO THE SYSTEM
There is a system feature call "info display" which is not defaulted to active. If it was turned on, a code had to be assigned to it. If that code is unknown then tech level programming can determine if it's on and what the dial code for that feature is.