Answer Calls
If you have selected the option "Alert me when I receive a call" in Calls preferences you will see the Incoming Phone Call Window when receive a call with connected phone.
The Replay button picks up the incoming call. The Reject button sends the call to voicemail or it send the busy signal it depends of which phone do you use, the Sending SMS... button to send a SMS message and Add Number... button if the name or number of caller isn't stored in your Address Book.