To enable Do Not Disturb:

Press the Menu key, and then selecting Features > Do Not Disturb.

A flashing icon and text on the graphic display indicates that Do Not Disturb is on.

Calls received while Do Not Disturb is enabled are logged in the Missed Calls list unless configured to be forwarded to another party by your system administrator.  

To disable Do Not Disturb:

Press the Menu key and then select Features > Do Not Disturb again.

Enabling Do Not Disturb on shared lines disables ringing only. You will still see a visual notification of the call and have the option to answer the call.