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 using KDDI (au)'s network. New entrants need years to build nationwide base station coverage, so roaming agreements fill the gaps in the interim. Users typically do not notice when they are roaming.
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, call rates can be several to tens of times domestic rates, and data charges can reach several hundred yen per MB, creating the risk of "bill shock" with charges reaching tens of thousands of yen unknowingly. 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. Since the Great East Japan Earthquake, the Ministry has been working to institutionalize "inter-carrier roaming" that temporarily opens networks in disaster-affected areas. Carriers that normally compete would open their networks to each other during emergencies, prioritizing communication access. When switching carriers via MNP, checking differences in roaming coverage areas provides additional peace of mind.