Lately, some people may have seen an increase of SPAM calls from "Unknown" numbers. You can't easily block these calls since your phone does not recognize a specific phone # to block. Your mobile carrier may be able to block these "Unknown" callers, but you may have to pay a monthly fee for that service.
Shouldn't you be able to block these "Unknown" callers on your iPhone using a mobile app? There are a few apps that offer this capability, but how well do they work?
For many people, it may not be a good idea to block calls from "Unknown" since physicians will often block their phone # (caller ID) so their calls will come in as "Unknown."