A number block is a group of phone numbers classified by their leading digits. In Japan's telephone numbering system, the first digits determine the purpose, region, or service type, all systematically managed under the Ministry's Telecommunications Numbering Plan. Understanding number blocks lets you instantly identify the type of an incoming call from an unknown number.
The main number blocks are as follows. 0AB-J numbers (10 digits starting with 01-09) are landline phones, with the area code identifying the region. 090/080/070 (11 digits) are mobile phones - 090 is the oldest, 080 was added in 2002, and 070 was opened for mobile use in 2013. 050 (11 digits) is IP phone, not tied to any region. 0120 (10 digits) is toll-free, 0800 (11 digits) is freephone - both are caller-pays-nothing numbers. 0570 (10 digits) is Navi-Dial, a caller-paid unified number. 020 (11-14 digits) is for M2M (Machine to Machine) communications, assigned to IoT devices.
Number blocks are managed by the Ministry and operated through number allocation to carriers. Carriers assign individual numbers to subscribers from within their allocated blocks. New block openings are considered as demand changes - for example, the opening of the 070 block for mobile phones was a response to exhaustion of the 090/080 blocks. The opening of the 060 block is also under discussion.
In practice, number block knowledge directly aids nuisance call defense. Calls from 050 numbers have a relatively high probability of being sales or fraud calls, and calling back a 0570 number incurs charges. International numbers starting with + should be handled cautiously, including the possibility of caller ID spoofing. See Phone Number Structure Guide for the overall numbering system and 090/080/070 Number Differences for mobile block details.