A withheld number (non-notification call) is a call where the caller's phone number is hidden from the recipient's device. In Japan, dialing "184" before the number activates this feature. Conversely, dialing "186" forces number display when the line default is set to withhold.
The mechanism relies on the Caller ID service in the telephone network. When "184" is prefixed, the exchange suppresses number information from being sent to the receiving end. However, the originating number is still recorded within the carrier's internal network, meaning law enforcement can trace it through proper legal procedures. "Withheld" does not mean "completely anonymous."
Common reasons for withheld calls include phone scam groups hiding their identity, telemarketers bypassing call blocking, and stalkers concealing their number. However, some legitimate organizations such as hospitals and corporate switchboards also use withheld settings by default.
Countermeasures include NTT's "Number Request" service, which plays an automated message asking withheld callers to reveal their number. On smartphones, iPhone's "Silence Unknown Callers" and Android's call blocking settings can filter withheld calls. See how to block nuisance calls for more options.