I don't know of a carrier that does this. Verizon has made it clear to me that in order to block a caller, you have to have a number to block.
You can try and create a contact card from a private or unknown caller. When such a call is received, tap the i in recents. Press on the name "Private" and copy. Then paste it in the contact card in the number field. Then block that contact. The downside is that may block a legit call.