Three choices. Roaming on your home plan is the simplest and usually the dearest, unless your carrier includes the country. A local SIM is cheapest for a long stay but means a counter, a passport and a swapped card. A travel eSIM sits between: bought online, installed before you fly, working as the wheels touch down. For a week or two the eSIM wins, and if your phone has no SIM tray it is the only choice besides roaming.
Install at home, activate on landing. Buy the plan a day or two before departure, scan the QR code on home Wi-Fi and leave the new line switched off. On arrival, turn it on, set it as the data line and switch data roaming off for the home SIM. Keep the home SIM on for calls and texts: bank codes and airline updates arrive on the number you already have while the eSIM carries the data.
Japan and Korea. Japan’s data-only eSIMs need no registration. Korea sells unlimited data-only eSIMs that activate from a QR code; passport verification applies only if you add a Korean number for calls and texts, and some plans must be activated within 48 hours of purchase. Both countries have SIM counters at the airports (Narita, Haneda, Kansai, Incheon).
Europe. If your SIM is from an EU country, roam like at home is law until at least 2032: your home allowance works in Lisbon, Rome, Berlin and Prague at no extra cost. It does not cover the UK, so check before London. Visitors from outside the EU get no such right; one regional eSIM covers Porto to Prague.
Bangkok and New York. Suvarnabhumi’s arrivals hall sells tourist SIMs quickly and cheaply, though a country eSIM avoids the line there and everywhere else. US roaming is often the dearest line on a phone bill, and an eSIM costs a fraction of it.
How much data. Maps are light: an hour of turn-by-turn navigation is around 5 to 10 MB, so a week of walking and metros rarely passes 300 MB. The photo library is what eats a plan; a day’s photos backing up over mobile data can be a gigabyte. Set backups to Wi-Fi only and a 5 GB week is comfortable; leave them on and buy 10.
On the day. Open the day’s plan on the hotel Wi-Fi before you leave, and use Download for offline in Waystop so the plan, tickets and documents work without signal. Only the live map, Chat and the taxi app need data. If the eSIM will not connect, toggle airplane mode and check which line carries data.