I wouldn’t open the system up to allow users to arbitrarily add numbers that hadn’t called as that
could would lead to people gaming the system.
This data could then be then shared up to the cloud, processed, and build a fairly-accurate list of telemarketers and spammers that could be redistributed to users to mark incoming unknown calls.
This data could also be shared with the CRTC, FCC and other governmental organizations who manage do not call lists for verification. For example, if I added myself to the do not call list and ABC company calls me, the appropriate agency could be automatically notified.
Obviously, this wouldn’t stop companies that fake call display / CallerID, but it would certainly help, but at this point, anything would be great. Currently about sixty percent of calls I get on my main number are telemarketing/spam/scam calls, so anything that Apple – or anyone else – can do about it would be fantastic.