A Directory Number is a number that is setup in your system to route to that will take you outside of the TRACK Pulse system. These numbers setup can be selected within Call Plans and Menus. Agents can select a directory number to transfer a call to. If the call is transferred to a directory number it will take them outside of the system and not require the caller to log into the agent dashboard to receive the phone call. This also means that you can not create a lead while on the phone.
See How do I setup a Directory Number?