The short answer: Using a travel eSIM rarely goes wrong at the QR code step. It goes wrong afterwards, when your home line is still awake and you have no idea what is eating your allowance. Five switches decide the outcome, and all of them live in your phone's settings: incoming calls on your home line, which line carries data, data roaming, Allow Cellular Data Switching, and Wi-Fi Assist.
Using an eSIM abroad begins after activation
Installing an eSIM profile does not finish the job. What you have added is one more line. Which line carries your data, and which line rings when someone calls, remain separate settings. Leave them untouched and you fly out with your home line running in parallel with a perfectly healthy eSIM.
Apple states plainly that on a Dual SIM iPhone only one number can use cellular data at a time. Data does not migrate to the eSIM by itself; you choose. The installation steps themselves are covered in setting up an eSIM for travel, so this article picks up where that one ends.
Your home line bills you for incoming calls
The voice side is the part travellers forget. NTT Docomo's official roaming pricing page states that overseas, a charge applies even when you receive a call. Domestic intuition says incoming calls are free; while roaming, the receiving side is charged. Turning data roaming off does not prevent this, because this is voice, not data.
The fix is simple. If you do not need calls to your home number during the trip, turn that line off in your line settings. If you do need to know who called but not to pay for it, arrange for calls to go straight to voicemail before you leave. Neither can be arranged from the departure gate, so do it while you are still home. How to keep your number available while travelling is covered in Dual SIM setup for travel.
Pin the data line to your eSIM
On iPhone, go to Settings, then Cellular, then Cellular Data, and select the eSIM you bought for the trip. Leave it on the home line and your traffic still travels the roaming path even though the eSIM is active and connected.
At the same time, turn data roaming off on the home line. Apple explains that data roaming lets you access the internet over a cellular network when you are outside your carrier's network, and that turning it off while travelling avoids roaming charges. On the travel eSIM, however, data roaming often needs to stay on, because most travel eSIMs are delivered by roaming onto a local operator's network. The correct configuration is asymmetric: off on the home line, on for the eSIM.
Allow Cellular Data Switching quietly brings the home line back
Even with the above set correctly, one path remains open. Apple documents that Allow Cellular Data Switching lets a voice-only line use voice and data during a call, and that the data line switches automatically for the duration of that call.
Abroad, that means placing or taking a call on your home line can pull your data over with it. Open Settings, then Cellular, then Cellular Data, and turn the option off. A single pre-departure pass over every relevant toggle is listed in phone settings before travelling abroad.
Your eSIM allowance is decided by what you watch
How much data you need is a function of use, not of days. Published figures make the spread obvious.
Netflix documents that on mobile the Automatic setting gives roughly four hours of viewing per gigabyte, and the lower data setting roughly six hours. At the highest setting, it notes that depending on device and connection speed, twenty minutes of viewing can consume a gigabyte or more. The same film can cost you an order of magnitude more or less, purely by setting.
So whether a travel eSIM lasts is not decided by the number on the plan alone. Dropping video quality one step before you leave changes the whole premise. Maps are the same story: download them in advance rather than streaming tiles all day, as covered in using offline maps.
Wi-Fi Assist is on by default
There is one more setting that spends your allowance without announcing itself. Apple states that Wi-Fi Assist is on by default, that it automatically switches to cellular data when Wi-Fi connectivity is poor, and that as a result more cellular data may be used, which can incur extra charges depending on your data plan.
Hotel and airport Wi-Fi abroad is very often connected but slow, which is exactly the condition Wi-Fi Assist reacts to. If your eSIM allowance is falling while you believe you are on Wi-Fi, check this first. The toggle sits at the very bottom of the Cellular settings screen.
Stop the traffic you are not watching
Apple describes Low Data Mode as pausing automatic updates and background tasks when the iPhone is not connected to Wi-Fi. Photo sync and app updates are the classic offenders on a trip: nothing you asked for, all of it billed to your travel allowance. Turning the mode on for the duration of the trip is the blunt and reliable answer.
Track your eSIM usage with the phone, not the app
Most travel eSIMs report remaining data only through the provider's app or portal. Your handset gives you a second, independent number.
On iPhone, Settings then Cellular shows cellular data usage per app, and on a Dual SIM device it shows usage for the number you selected for cellular data. That counter does not reset on its own, so as Apple notes, use Reset Statistics at the bottom of the screen. Reset it the day you fly and everything you see afterwards is your trip.
Android goes further. Google documents a data warning and a data limit: set the limit and, when it is reached, mobile data turns off automatically, a notification appears, and you have to switch it back on by hand. Set the limit slightly below your purchased allowance and you cannot silently overrun it.
When it is slow rather than exhausted
If data remains but speed does not, the problem is usually the connection, not the allowance. Check which network the eSIM has attached to, confirm data roaming is on for that line, and force a re-registration with airplane mode. The full diagnostic order is in what to do when your travel eSIM is slow.
FAQ
Should I delete my home line once the travel eSIM is installed?
No. Deleting it means setting it up again when you get home. Turning the line off, or turning off its data roaming, is enough for the trip. If you also want no incoming calls, turn the line itself off.
Should data roaming be on or off?
Decide per line. Off on your home line, on for the travel eSIM. Travel eSIMs are usually delivered by roaming onto a local operator, so switching roaming off on the eSIM itself leaves you with no connection at all.
Why is my data dropping while I am on Wi-Fi?
Most likely Wi-Fi Assist. Apple states it is enabled by default and switches to cellular data automatically when Wi-Fi connectivity is poor, which can increase cellular data usage. You can turn it off at the bottom of the Cellular settings screen.
How many gigabytes should I buy?
Decide by use, not by days. Maps, search and messaging need very little. Video dominates everything else, and quality settings dominate video: Netflix's own figures give about four hours per gigabyte on Automatic, versus a gigabyte or more per twenty minutes at the highest setting.
Can I leave an expired eSIM on the phone?
An expired profile does not generate charges, but it clutters your line list and makes it easier to pick the wrong line next time. Clear it out before your next trip; the steps are in deleting an eSIM after your trip.
Five minutes before departure decides what your eSIM is worth
Every setting above lives in the phone's own settings screen and the whole pass takes a few minutes. It matters out of proportion to the effort because what you lose by skipping it is invisible: charges you did not authorise and allowance you did not spend on purpose. Neither is recoverable once you notice it abroad. Before you fly, walk through the five in order: incoming calls on the home line, the data line, data roaming, Allow Cellular Data Switching, and Wi-Fi Assist. Plans for your destination are at Coral eSIM plans.