A spam call filter is a feature that cross-references incoming caller numbers against a database, displaying warnings or automatically rejecting calls identified as potential nuisance calls. Unlike manually setting call blocking, it cross-references a database that aggregates a large volume of number reports, so numbers you have never seen can also be flagged.
Carrier-provided services include Docomo's "Anshin Security," au's "Spam Message/Call Block," and SoftBank's "Nuisance Call Block." Pricing, how each one bundles with other options, and supported handsets differ by carrier and plan, so check with your provider before subscribing.
Among third-party apps, Whoscall and Denwa-cho Navi are widely known; they identify the caller during an incoming call and display a warning when the number is suspected of being a nuisance call. What each app offers for free and what a paid plan adds differ by app and can change with updates, so check each app's own information before installing. Denwa-cho Navi collects posts from its users, so you can look up reports about domestic sales calls and numbers suspected of being scams.
Spam call filters are not perfect and may incorrectly block legitimate calls (false positives). A newly opened store's number might be registered as a former nuisance number, or a call center number might be flagged as telemarketing. When expecting important calls, consider temporarily relaxing filter settings. Most filter apps record blocked numbers in call history, so regularly checking for false blocks is a good habit. See how to use spam call databases for the steps to look up a number and how to read the results.