Google has tons of verified public phone numbers for its map products. Android should be able to do reverse lookups of unknown callers. This might not work for

Google has tons of verified public phone numbers for its map products. Android should be able to do reverse lookups of unknown callers. This might not work for unlisted telemarketers. To catch those numbers, perhaps Google could anonymously collect Android users' phone blocklists. That does not have quite the same privacy concerns as sharing your entire contacts list with Google. This is the phone number version of Gmail's spam filter.