Check your telecom provider, they may have a block call list available to you. If you have Charter do this:
Dial *60
Press 1 to enable the charter call block list
Then press 2 to add a number to the list
Type in phone number without international code "1", just XXX-XXX-XXXX then press the # sign
Press 6 to exit the call
After about a dozen numbers the list is full and you can't add anymore. Scroll through the call log on your phone and write down numbers from the most recent to the past. Dial into the call block service again, press 4 to clear the list, then press 2 and add each number starting from the most recent backward. This keeps your block list up to date.