If you use your computer to recieve phone calls but don't use the VoIP system, you will find PhoneTray particularly helpful, as it displays the caller's phone number.
You can also configure PhoneTray Free to avoid calls from certain numbers by making the caller believe you left the phone off the hook or that the number no longer exists. To do this you just need to record an audio in WAV format and PhoneTray will reproduce it for certain numbers automatically.
Call barring can also be based on the the following restrictions: No caller ID, private number, local or long distance number, etc.
PhoneTray Free can also, among other things, create a history of all the calls received on your PC, set different call tones and alarms depending on the caller's identity or hang up automatically.