Compared across countries

Can you use your own phone?

For a trip that crosses borders — the same question, answered country by country.

The pattern

Where the SIM is registered against your passport — China, Thailand, Vietnam — buying one is a small piece of paperwork you do on arrival. Elsewhere it is bought and activated in minutes: Japan and the United States both sell eSIM plans with nothing to register, and Korea and Taiwan run public wifi wide enough to lean on.

United States

Yes, if it is unlocked — and that is the one thing you cannot fix after you land. There is no passport registration to do here; a local eSIM plan is bought in an app in a few minutes.
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.

Sort the practical stuff before you fly.