If you own a call blocker model CT-CID803 version 2.0, N2 or 5.0, you can turn the SET 6 feature ON, to block all calls with incomplete or generic information in the caller id, including:
- Calls showing the phone number as "000-000-0000"
- Calls showing the phone number as "------------"

- Calls showing the phone number as "PRIVATE"

- Calls showing the phone number as "--P--" (P means Private)
Calls showing the phone number as "Out Of Area"

Calls showing the phone number as "Unknown"

- Other variations may be blocked depending on your carrier.

For details on how to turn ON or OFF this feature, please refer to the manual.
If you are not sure which version you have, please refer to this page.

When I turn SET 6 ON, some calls from my friends and family are blocked. Why? 
Unfortunately many carriers in the USA & Canada, especially cell phone carriers, do not send the phone numbers as part of the Caller ID information, they send the caller's phone numbers as part of the ANI (Automatic Number Identification).  On the other end, the TELCO equipment at the Local Exchange Carrier (LEC) do not convert the ANI into the Caller ID.  The end result is that when this feature is ON, it may block many legitimate calls from friends and family, even if they show a phone number in the  LCD display of the call blocker.
Recommendation: Only turn the SET 6 feature ON periodically when you are getting bombarded by unwanted calls.