Unlike in the iOS and Android applications, if you have quit the desktop app, you will not receive any calls through it.
Incoming calls will sound according to your Audio Source Settings. Click the answer button to answer the call or the red button to decline the call.
Some Jabra headsets also allow you to answer calls by clicking a button on the headset itself or by picking it up.
Click on the red button when a call is coming in to decline the call. Cradle will either find someone else to answer the external call based on Roles or send it to Voicemail.