The directory gives you a handy list of important numbers that you can call directly from your device.
Tapping a contact to will show a drop down menu with a list of the different numbers for that contact. These include:
internal extensions (direct dial from device)
external numbers (direct dial from device)
To make a call just tap on the number you want to call.
Save a Number
You can save your most useful contacts by tapping the star to the right of the number.
Then go to the 'Saved' screen on the app to see your saved contacts. To un-favourite a contact, just tap the star again.