Roaming is a system that allows you to make calls and use data on a partner carrier's network when you are outside your own carrier's service area. The word comes from "roam" (to wander), and the purpose is to maintain connectivity as users move across areas.
There are two types of roaming: domestic and international. Domestic roaming connects to another carrier's network in areas where your carrier has no base stations - a prime example is Rakuten Mobile, which has been using KDDI (au)'s network through roaming. New entrants need years to build nationwide base station coverage, so roaming agreements fill the gaps in the interim. While roaming, users keep making calls and using data with the same phone and number.
International roaming automatically connects to a local carrier's network when traveling abroad. With "Data Roaming" enabled in device settings, the phone connects to the local network upon arrival at the airport. However, using it without a flat-rate plan can become expensive, creating the risk of "bill shock" from unexpectedly high charges. Countermeasures include subscribing to an overseas flat-rate data plan before departure, purchasing a local SIM card, using an eSIM overseas plan, or leveraging Wi-Fi Calling.
Disaster roaming also plays an important role. Following a Ministry of Internal Affairs and Communications study group that began in September 2022, Japan's four mobile network operators launched "emergency inter-carrier roaming" on April 1, 2026, letting users temporarily use another carrier's network when their own is unavailable due to a major disaster or network failure. Carriers that normally compete open their networks to each other in emergencies, within the spare capacity of the rescuing carrier. When switching carriers via MNP, checking differences in roaming coverage areas provides additional peace of mind.