Use Anonymous Call Rejection to stop calls from ANONYMOUS and PRIVATE numbers. For instance, if a sales guy calls you from a blocked number, he hears:
The party you are calling does not accept blocked calls. If you are calling from a blocked number, please hang up, pick up the receiver, press *87 and redial the phone number. When you do this, your name and number will appear for this call only.
In many areas, Anonymous Call Rejection comes with Caller ID. In some areas, you need to order it separately. Either way, when it's first installed, it's turned OFF, so you need to turn it ON.
This calling feature only stops ANONYMOUS and PRIVATE calls. Other types of calls that lack the info needed for caller ID, such as OUT OF AREA, UNAVAILABLE, UNKNOWN or NOT PROVIDED, cannot be blocked with this feature.