Call blocking is a feature that automatically blocks incoming calls matching specific phone numbers or conditions. As the most basic and immediately effective nuisance call countermeasure, it can be configured at three levels: device, carrier, and app.
In smartphone device settings, specific numbers can be individually blocked from call history or contacts. On iPhone, use "Block this Caller" in the Phone app; on Android, use "Block number." Blocked numbers won't ring, and callers hear a busy tone or announcement.
Carrier services include Docomo's "Nuisance Call Stop Service" (free, up to 30 numbers), au's "Nuisance Call Rejection Service" (110 yen/month), and SoftBank's "Number Block" (110 yen/month). Unlike device settings, these block at the network level, remaining effective even when the device is powered off.
For landlines, NTT's "Nuisance Call Rejection Service" (660 yen/month) is available. Registering the most recent incoming number causes subsequent calls from that number to hear "This call cannot be accepted" and disconnect automatically. Up to 30 numbers can be registered. Many phone models also have built-in call blocking features, and combined with Number Display, can blanket-reject anonymous or unregistered numbers.
Third-party spam call filter apps (Whoscall, Denwa-cho Navi, etc.) can automatically identify and block nuisance numbers registered in cloud databases. Review detailed setup procedures by device in how to block nuisance calls.