Number exhaustion refers to the problem of running out of available phone numbers. Phone numbers are a finite public resource, and demand has repeatedly outstripped supply as telecommunications services have expanded. In Japan, the explosive growth of mobile phones has been the primary driver of number exhaustion.
The history of mobile number exhaustion is also a history of expanding number blocks. In 1999, mobile numbers were lengthened to 11 digits, and in 2002 the 080 block was added. In 2013, the 070 block was opened for mobile phones following the decline of PHS services. The combined capacity of the 090, 080, and 070 blocks is approximately 270 million numbers, and as of 2026 roughly 265 million of them have already been allocated to carriers, an allocation rate (numbers allocated divided by the total capacity of roughly 270 million) of about 98% - the blocks are close to fully used. Mobile subscriptions, a separate measure, stood at approximately 233 million at the end of March 2026 (Telecommunications Carriers Association figures); allocated numbers and subscriptions are not the same thing. With growing demand for IoT device numbering, the 060 block has been formally added for mobile phones by a December 2024 amendment to Japan's numbering plan (the launch, originally planned for July 2026 onward, has been postponed and had not started as of August 2026).
Landline numbers have also experienced exhaustion. In major metropolitan areas like Tokyo (03) and Osaka (06), population concentration and corporate density have strained area code capacity. Countermeasures have included changing the number of digits in local exchange codes (Tokyo's local codes expanded from 3 to 4 digits) and assigning new area codes. However, subscriber telephone and ISDN lines have fallen from roughly 63 million in fiscal 1997 to about 14 million in 2023, so the situation is less critical than for mobile numbers.
The key to addressing number exhaustion lies in flexible management of the number allocation system. The main tools are recovering unused numbers (returning allocated but unused blocks), promoting number recycling (reassigning canceled numbers), and opening new number blocks. The 020 block was established in 2017 for M2M (Machine to Machine) communications, with 11-to-14-digit numbers assigned to IoT devices. See History of Phone Number Digit Lengths for the evolution of the numbering system.