Phone number recycling is a system where canceled phone numbers are reassigned to new subscribers after a cooling-off period. Phone numbers are a finite resource, and in urban areas where number exhaustion is advancing, reusing unused numbers is unavoidable. Carriers manage the cooling-off and reassignment process under the Ministry's Telecommunications Numbering Plan.
When you receive a recycled number, calls and SMS intended for the previous owner may arrive. This is especially serious when the previous owner registered the number with various web services. Two-factor authentication codes could be sent to the new owner, potentially granting access to the previous owner's accounts. Conversely, if the previous owner had debts, debt collection calls may repeatedly reach the new owner.
Carriers set cooling-off periods before reassignment, but the duration varies. NTT uses approximately 6 months for landlines, and mobile carriers generally observe 6 months to 1 year. However, for the heavily exhausted 090 number block, cooling-off periods tend to be shortened, increasing the risk of reassignment before the previous owner's traces have fully cleared.
As a precaution before canceling a number, it is important to update the phone number on all registered services. Banks, social media, e-commerce sites, and services with two-factor authentication are top priorities. If the new owner continues to receive calls for the previous owner, they can contact their carrier to request a number change. Using number portability to keep your number when switching carriers avoids the recycling issue entirely. See Phone Number Privacy Tips for specific measures.