Area Code 4240 – Kokubunji, Tokyo

Overview of Area Code 4240

Area code 4240 is a geographic National Destination Code (NDC) within the Japanese numbering plan, assigned to the city of Kokubunji in the western part of the Tokyo Metropolis. This code is part of the broader 042 area code cluster, which serves multiple cities in the Tama region, including Kokubunji, Koganei, and Tachikawa. When dialing within Japan, the area code is typically written as 042-40, but the full international format includes the country code and the leading zero is dropped.

Geographic Coverage and Administrative Jurisdiction

Kokubunji is a city located in the western portion of Tokyo Prefecture, approximately 25 kilometers from the central wards of Tokyo. It is a residential and commercial hub with a population of over 120,000. The city is known for its universities and research institutions, such as the Tokyo University of Agriculture and Technology. The 4240 prefix is used for fixed-line telephone services within this area, managed by NTT East, the regional incumbent carrier.

Dialing Formats and Caller Instructions

For local callers within the 042 area code region, dialing a Kokubunji number requires only the subscriber number, which is typically 7 digits long. For example, a local call to a number like 042-40X-XXXX would be dialed as 40X-XXXX. However, when calling from outside the 042 area code but still within Japan, callers must dial the area code including the leading zero: 042-40X-XXXX. For international callers, the number must be dialed in the full E.164 format: +81 42-40X-XXXX, where the leading zero is omitted. For instance, a typical Kokubunji number might be +81 42-401-2345.

Overlay and Numbering Considerations

As of the latest numbering plan updates, the 042 area code does not have an overlay code; it remains a single NDC. However, due to the growth of telecommunications services, some numbers within the 042 area may have been reassigned or ported. Number portability is available in Japan, allowing users to keep their numbers when switching carriers, but the geographic significance of the code remains tied to the physical location of the line.

Practical Information for Callers

When calling a Kokubunji number from abroad, it is essential to use the correct country code (+81) and to omit the leading zero of the area code. For example, if the local number is 042-424-0000, the international format is +81 42-424-0000. Charges for international calls vary by carrier, but calls to geographic numbers are typically billed at standard international rates. For businesses or individuals in Kokubunji, having a 4240 prefix indicates a fixed-line connection, which is often used for reliable voice services, fax, and ADSL or fiber-optic internet connections.