When a call is coming in, in addition to hearing the phone ring, you can see which contact is calling on the right sidebar. In case of a new contact, the phone number is displayed.
If you wish to take the call, click the “Green” button on the right hand sidebar. If you're busy and cannot take the call, click the “Red” button.
If you decline the call, depending on your Routing settings, the call might be diverted to another user, your voicemail or an external phone number, depending on how your administrator configures these settings.