Can you use your own phone?

Japan

Yes. A prepaid eSIM is usually far cheaper than roaming — but it gives you data only, not a Japanese number.

China

Yes, but decide how before you fly. A local SIM has to be registered against your passport, and a number of services you use daily will not load.

Thailand

Yes, if the handset is unlocked — and bring your passport, because every SIM here is registered to a named person.

Vietnam

Buy a prepaid SIM on arrival with your passport — it is cheap, the coverage is good, and a local number is what makes the ride-hailing apps work.

South Korea

Yes, and you may barely need data — free public Wi-Fi runs on subway trains, buses and in most cafés.

Taiwan

There is a genuine national free wifi network — iTaiwan, over 5,000 hotspots, and since 2020 no registration at all. Connect, open a browser, done.