D-Link DPH-150S User Manual page 87

7.1.6. DND
DND means Do Not Disturb from any other call. If you want to have this function, please press [DND] function key; if you don't want to have this function,
press [DND] function key again.
NOTE:
If DND function is active, LED1 indicator will be turned on, in this state, you can call anyone or everybody can't call you.
87