Try calling your carrier. it may just apply to landlines but when I had a landline before I went to just my cell, I had that option on my house phone.
Do you have verizon by any chance? Not sure if this works but
http://www.ehow.com/how_5882329_block-unknown-callers-verizon-wireless.html
Instructions
1
Navigate to vzw.com. Click "Sign In" in the upper right corner of the page. Log in to your Verizon Wireless account using your username or wireless phone number and online password.
2
Select your phone number from the list if your account's phone numbers. You'll find this list under "My Cell." Click "Go."
3
Click "Verizon Safeguards" in the menu on the left-hand side of the page, then click "Parental Controls."
4
Click the tab marked "Usage Controls." Scroll down until you see the menu for "Blocked Numbers." Click on "Blocked Numbers."
5
Click on "Add Blocks." Select the box next to "Restricted, Unavailable, and Private Numbers" and click "Ad
Read more:
http://www.ehow.com/how_5882329_block-unknown-callers-verizon-wireless.html#ixzz2un90qQbj