We don't have a feature to block "unidentified" callers at this time. We can block callers that block their number, a specific number, or you can block all numbers and allow up to 30 to call you. There are devices you can purchase that can manage the calls.
You can buy one of the phones that allow you to block up to 300 numbers. Most of those phones allow you to block "unavailable" or "unknown" numbers. Cox offers a Selective Call Rejection feature but it is very limited and will not block "unavailable".