Number Request is an NTT service for landlines that plays an automated message to withheld-number callers, asking them to redial with caller ID enabled (by prefixing 186). Available for 220 yen/month (tax included).
The key benefit is automatic separation of legitimate and nuisance calls. Genuine callers will redial with their number shown, while nuisance callers and scammers who fear identification rarely comply.
Most effective when combined with Number Display - verify caller IDs on displayed calls while blocking withheld ones, filtering out most suspicious calls.
Limitations: public phone calls and some international calls cannot display numbers and bypass this service. Caller ID spoofing with fake numbers also circumvents it.
Mobile phones lack an equivalent service but have built-in withheld call rejection: iPhone's "Silence Unknown Callers" and Android's "Block unknown numbers."