Number portability is a general term for systems that allow switching carriers or services while keeping your phone number. MNP (Mobile Number Portability) for mobile phones is the most widely known, introduced in Japan in October 2006.
Mobile MNP works in two ways. Under the older two-stop method, you obtain an MNP reservation number from your current carrier before signing a contract with the new one. Under the one-stop method, the destination carrier handles the switch on its own. The one-stop method started on 24 May 2023, but it is limited to web procedures between carriers that both support it, so the reservation-number route remains a current option. Transfers handled on the web are free, while procedures done by phone or at a store may involve a fee (per Ministry of Internal Affairs and Communications guidance as of August 2026).
Landline number portability exists too, though it is less flexible than the mobile version. A number can sometimes be carried over when moving from an NTT subscriber line to Hikari Denwa, or when changing providers within the same area code zone. Whether it carries over depends on how the number was originally issued and on facility conditions, and there are cases where it cannot be used as is. Numbers generally cannot be carried over when a move changes the area code, or when the number was first issued by a provider other than NTT.
Number portability is best understood as a rule that lowers the barrier to changing providers. Because you do not have to give everyone a new number, it is easier to compare prices and services before switching, and the same process is used when moving to an MVNO (budget SIM). What people overlook is that the number itself carries over while the things attached to it may not. A carrier email address is handled separately: keeping it means signing up for a paid service, and the window for applying is limited (conditions differ by carrier). Fixed-line number portability (LNP) is used when migrating from NTT to Hikari Denwa or switching to another fiber phone service. See MNP - how it works and how to switch for the steps involved.