In order to call a colleague or an external contact, you can search for the specific keyword in the search field. The best result is automatically placed at the top of the list and you can directly call the required extension using one of the enter on your keyboard.
If you want to ring directly to the mobile telephone, to voice mail or through a notification, you can either right-click on the extension and select the option under Call. All of these commands have hot keys. A complete list of the hot keys are available in the help menu which you can access by pressing F1.