Select Participant window
If you click the "+" button at the end of the "To" box to add participants, you'll see the Select Participant screen. It lists the different users, extensions and supported devices connected to the telephony system.
Type
Select which type of participant to display from All, Extension, All devices or User.
Search text
Enter the text to filter the search results by. To show every participant of the selected type just leave this box blank.