Most IP phones have a built in DND (Do Not Disturb) feature.  If this feature is toggled "ON" from the phone, any call directed at that phone will return an unavailable message to the PBX, which usually causes the call to go directly to Voicemail.

You can now use use the Protel LTS PBX to handle DND status instead, which gives you the added benefit of alerting other users when your phone is in an unavailable status.

To enable this feature in Protel LTS:
Make sure your PBX has the latest update release.  This should happen automatically when you log in as an administrator, or it can be run manually under the Maintenance menu.
Go to the "Modify Feature Codes" page, located under the Additional Tasks Menu
Click on the box at the top to reset all feature codes to default, then click on the Save button.  NOTE if you have modified any of the default feature codes, you will need to go back and reset those to your desired values.
By default, the feature code for DND will be set to the text value of "dnd".  This can be changed to a numeric value at your option, which you will need to do if you are not assigning a BLF button on phones for the DND feature, since an alphabetic feature code cannot be dialed directly from a phone keypad.
Make sure that you do a dialplan reload after your changes have been made

TO PROGRAM PHONES MANUALLY:
For the key that you wish to assign as the DND, simply make it a BLF button and enter dndxxx in the field, where xxx is the same extension number that you are programming buttons on.  Or, if you have changed the feature code to a numeric value, substitute that value for the dnd text.
Now, pressing that button on the phone will register it as being in a Do Not Disturb state, and the button that you programmed will light up to notify you that the phone is set in DND.  Additionally, any other phone that is monitoring that extension on a BLF key will display that phone as being unavailable.

TO PROGRAM PHONES AUTOMATICALLY:If you are using the phone configuration files in the PBX to auto-provision your end points, you will notice that there is now an added button type available called "DND".  By assigning a button to this type, you will cause the button to provision as a PBX-side DND.

USAGE NOTE:If you use PBX-side management of DND, keep in mind that any extension that is placed in a DND state will still be available to receive calls if they are part of a ring group.  If you need the phone to be unavailable for ALL calls, including group rings, you will still need to use the phone-side DND.