I think what you are looking for is an individual station option. I found this procedure in the DT300/700 user guide.
• TO ENABLE/DISABLE BACKLIGHT
The following explains how to enable/disable the
backlights of LCD and digit keys.
The backlights illuminate (for approximately 10
seconds) when you press any key or lifts the handset.
STEP 1: Display the Menu screen, and select Setting.
STEP 2: Select User Setting.
Select an item by either of the following operations.
• Highlight an item by using , and then press ,
or OK soft key.
• Press key.
STEP 3: Select Display.
Select an item by either of the following operations.
• Highlight an item by using , and then press ,
or OK soft key.
• Press key.
STEP 4: Select Back Light.
Select an item by either of the following operations.
• Highlight an item by using , and then press ,
or OK soft key.
• Press key.
STEP 5: Make a selection from the following list.
Select an item by either of the following operations.
• Highlight an item by using , and then press
or OK soft key.
• Press or key, and then press or OK soft
key.
Menu Item Meaning
1 Disable
Backlights of LCD and digit keys
are disabled.
2 Enable
Backlights of LCD and digit keys
are enabled.
Note: The enabled item is highlighted.