Go to and make sure that your desk phone's selected. If you see My Computer instead of your desk phone, select it and switch to your desk phone.
Dial a number from your app, and your call goes through your phone. Pick up your handset, use your headset, or talk over your speakerphone on your phone. Then continue controlling your phone from the app to put calls on hold, and merge, conference, and transfer them too. You'll see everything taking place on the phone display.