The short answer, in one screen
Set your travel eSIM as the mobile data line with data roaming on, and leave your home SIM active for calls and texts with its data roaming off. Then turn off Allow Cellular Data Switching so your phone can never fall back to your home number for data.
That is the entire configuration. It feels fiddly because the two lines share setting names with opposite correct values, and one switch hides outside both lines. The table below is the setup; the sections after it explain each row.
| Setting (iPhone) | Home line | Travel eSIM | What goes wrong if you flip it |
|---|---|---|---|
| Turn On This Line | On | On | Off at home means no bank texts. Off on the eSIM means no data. |
| Mobile Data (the data line) | Not selected | Selected | Pick the home line and every app spends your carrier's roaming data. |
| Data Roaming | Off | On | The usual reason a new eSIM "doesn't work". |
| Default Voice Line | Your home number | Not used for voice | Outgoing calls leave from a number nobody recognizes. |
| Wi-Fi Calling | On, if your carrier offers it | Leave alone | Losing your home number over hotel Wi-Fi. |
Two settings sit outside that table because they apply to the whole phone: Allow Cellular Data Switching, which belongs off, and Start New Conversations From, which should point at your phone number.
Which SIM does what: data, calls, texts
A dual SIM phone doesn't switch between lines the way a laptop switches Wi-Fi networks. It runs both at once and routes each job separately. Data has a designated line, outgoing calls have a designated line, and incoming calls and texts arrive on whichever number the sender used.
| Job | Which line handles it | What you control |
|---|---|---|
| Mobile data: maps, apps | The line you set as the data line | Full control. Set it to the travel eSIM. |
| Outgoing calls | Your default voice line | Full control, adjustable per contact. |
| Incoming calls | The line that was dialed. While both are on, a call to either number rings the phone. | None, beyond turning a line off. |
| Incoming SMS, including 2FA codes | The line it was sent to: your home number | None. This is why the home line stays on. |
| WhatsApp, Telegram, Signal, email | Ride on the data line, so on the travel eSIM | Nothing. They follow data automatically. |
So "which SIM for data, which for calls" has a boring answer: data on the travel eSIM, voice on your home number, and no reason to change it mid-trip. Still weighing whether a second line beats paying your carrier? See our eSIM versus roaming breakdown.
Data roaming: one switch, opposite values
Here is the detail that trips up nearly everyone who writes out their own steps and asks strangers to check them. Data Roaming is a per-line setting, and a correct setup needs a different value on each line.
Data Roaming belongs OFF on your home line and ON on your travel eSIM. Same switch, same wording, opposite answers, one screen apart. Every other setting in this guide is a refinement of that one rule.
On the travel eSIM it has to be on. To the phone, that profile is foreign and connecting to a local partner network, so technically it is roaming even though roaming is the entire product. Leave it off and the line attaches, shows bars, and passes nothing.
On the home line it has to be off, for the mirror reason. That line keeps hunting for a foreign network so it can receive calls and texts, and roaming data left enabled is what lets a stray connection bill at your carrier's international rate. Treat it as the backstop to the next section: if data switching is on, or an update turns it back on, this switch still stops the charge.
Am I billed just for receiving a text?
Travelers argue about this endlessly, and both camps sound certain. What people commonly report is that incoming SMS costs nothing, while calls in either direction and outgoing texts can trigger a charge, sometimes a day pass that starts the moment the line is used. We won't state that as fact for your carrier: it is the kind of claim that turns out wrong for one plan.
Your carrier's international roaming terms are the only authority on your bill. Read them before you fly, watching for three things: whether receiving an SMS is chargeable, whether answering a call triggers a day pass, and whether that pass activates by itself.
Allow Cellular Data Switching should be off
iPhones have a setting called Allow Cellular Data Switching, tucked into the Mobile Data screen rather than into either line. It exists for a good domestic reason: when your data line has weak coverage, the phone borrows data from the other line. At home, helpful. Abroad, a way to spend money you didn't plan to.
It tends to arrive switched on, and at least one widely copied set of travel instructions claims it can stay that way. Don't. With it on, a patch of weak signal lets the phone fall back to your home number's data, at roaming rates, with nothing on screen to warn you.
Turned off, behavior becomes predictable: weak signal means slow data instead of surprise charges. This is the one switch that overrides your data-line choice on its own, so the rest of the setup only holds while it stays off.
The battery side effect nobody mentions
Travelers frequently describe a second problem with leaving both lines active abroad: the home line drains the battery searching for a network it can barely reach, which matches how any phone behaves in poor coverage. If your carrier has no partner there and your codes are handled another way, switching Turn On This Line off is legitimate.
Default voice line decides less than you think
Default Voice Line sounds like a master switch. It isn't. It decides which number your outgoing calls leave from, and nothing more. It does not stop the other line ringing: while both lines are on, a call to either number rings the phone, and there is no per-line control over incoming calls.
For most trips, leave it on your home number: people recognize it and callbacks reach you. Many travel eSIMs are data-only anyway, and a data-only line has no voice service to be the default for.
iOS also allows a per-contact override. Open a contact, choose Preferred Line, and calls to that person leave from the line you picked. Useful for a hotel you'll call repeatedly, if your plan includes voice.
The call forwarding trick
A sharper move some travelers arrange before leaving: they set their carrier's call forwarding to send incoming calls to an internet calling number, so the call arrives over data on the travel eSIM instead of a foreign network. Two caveats. Forwarding is a carrier feature, so availability and billing depend on your plan. And it must be set up while you still have home service.
Keeping your bank's 2FA texts alive
Strip away the menus and this is what most people are really asking. They need their bank abroad, the bank sends a one-time code by SMS to their home number, and some banks offer nothing else. Losing that number means being locked out of your own money in a country where you need it.
The configuration above solves it, and the mechanism is worth stating plainly: SMS arrives on the line it was addressed to, and that line only needs to be registered on a network, not to have data. Your home line keeps receiving codes; the travel eSIM carries everything else.
Three things still deserve a pre-flight check:
- Fraud alerts that expect a reply. Plenty of banks text you to reply YES or CONFIRM. That's an outgoing message on your home line abroad, a different billing question from receiving one. Check your carrier's terms, and register your travel dates so the alert is less likely to fire.
- Countries where your carrier has no partner. If the home line can't register at all, the one thing that may still deliver the code is Wi-Fi Calling, where your carrier allows it abroad. Don't lean on that alone: move critical accounts to an authenticator app or a passkey before departure.
- Anything you can move now. Switch accounts to app-based codes while you're home and can receive the confirmation text. Do it once and the problem shrinks permanently.
iMessage, FaceTime and your vanishing number
A second cluster of complaints is about identity, not data: everything works, then friends report messages from an unfamiliar address, threads turning green, or FaceTime not reaching the usual number.
Two causes. iMessage and FaceTime register your number through the line it belongs to, so if that line loses service long enough, or you deleted it to fit the travel eSIM, the number can stop being an available address and only your Apple Account email remains.
The second cause is quieter and easier to fix. In Messages settings, Start New Conversations From decides which address new threads use. If it flips to your email, everyone you contact for the first time abroad sees a sender they don't recognize. Set it back, then check again a day later.
So think twice before deleting profiles. Removing your home eSIM to fit a travel one isn't reliably reversible on every carrier. If slots are your constraint, read how eSIM and physical SIM differ in practice and find your model in the list of eSIM-compatible phones first.
The checklist: before you fly, on arrival, back home
Forum threads keep reinventing this list, and the best versions sit buried in comment replies. Here it is consolidated and split by moment, because the mistake is usually a right step done at the wrong time.
Before you fly, while you still have home service
- Confirm the phone is unlocked and eSIM-capable. A locked handset can refuse a second profile outright.
- Buy and install the travel eSIM at home over Wi-Fi. Installing is not activating on most plans, and a midnight install on hotel Wi-Fi is a bad start. Our step-by-step eSIM installation guide covers the QR and manual routes.
- Rename the lines Home and Travel. Every settings screen afterwards becomes obvious instead of a guess between two identical entries.
- Switch your home line's Data Roaming off.
- Move any 2FA you can to an authenticator app, and tell your bank your dates.
- If you want incoming calls without foreign networks, arrange forwarding now.
- Download offline maps and boarding passes, so landing with no data is a mild inconvenience.
On arrival, after you land
- Turn off airplane mode and give both lines a minute to register.
- Switch the data line to the travel eSIM.
- Switch Data Roaming on for the travel eSIM. This is the step people skip.
- Switch Allow Cellular Data Switching off.
- Confirm the default voice line is still your home number.
- Check Start New Conversations From points at your phone number.
- Load an uncached page to confirm real data, then have someone text your home number.
Back home
- Set the data line back to your home number.
- Turn the travel eSIM's line off, or delete the profile if the plan is finished.
- Restore your home line's Data Roaming if you normally keep it on.
- Leave Allow Cellular Data Switching off unless you actively want it back.
When a step fails, work down the list rather than starting over. The usual culprit is data roaming left off on the eSIM, the wrong line chosen for data, or a plan that only starts on first connection abroad.
Android and Samsung: same logic, different labels
Every rule above applies to Android. Only the wording moves, and it moves by manufacturer, version and sometimes carrier skin, so look for the concept rather than a fixed menu path.
| Concept | iPhone wording | Typical Android or Samsung wording |
|---|---|---|
| Which line carries data | Mobile Data, then pick the line | Preferred SIM for mobile data |
| Roaming permission, per line | Data Roaming, inside each line | Data roaming, inside that SIM's settings |
| Borrowing data from the other line | Allow Cellular Data Switching | Automatic data switching |
| Which number calls leave from | Default Voice Line | Preferred SIM for calls, or Ask every time |
| Turning one line off | Turn On This Line | The SIM's toggle in SIM manager |
Two Android notes. Some phones offer "Ask every time" instead of a fixed default, which is clearer on a trip as long as bank replies go out on the home line. And several hide automatic data switching in a submenu that only appears once two lines are active, so configure it after installing. New to all this? Start with what an eSIM is and how it changes traveling.
When a travel eSIM is not the right answer
Fair beats persuasive here. A travel eSIM is not automatically the right call, and on some trips this setup is more effort than it saves.
| Your situation | Better choice | Why |
|---|---|---|
| Your plan already includes usable data at your destination | Use your home plan | One number, no settings, nothing to install. Confirm the country is genuinely included. |
| A short hop, mostly spent on Wi-Fi | Wi-Fi plus offline maps | Not every trip needs a mobile connection to justify the setup. |
| You need a local number for calls, deliveries or local apps | A local physical SIM with voice | Most travel eSIMs are data-only. A callable number is a different product. |
| A carrier-locked phone, or one with no eSIM support | Local SIM, or an unlocked spare handset | No configuration overcomes a locked device. |
| Weeks of heavy data use in one country | Compare a local long-stay plan | Local carriers sometimes win on volume. Compare before assuming. |
When a travel eSIM does fit, which covers most short and medium trips, the appeal is that it slots into the configuration above without touching your home number. PuraSIM covers more than 200 destinations, activation takes about a minute, and you have 180 days to activate after purchase, so buying early costs no flexibility. If something misbehaves at night, local time, support answers by email and WhatsApp, 24/7, in every language. Find your destination in popular travel destinations.
Frequently asked questions
What settings should my phone be on when I travel internationally?
Four things. The travel eSIM selected as the mobile data line. Data roaming on for that eSIM, off for your home line. Allow Cellular Data Switching off, so the phone can't borrow data from your home number. Default voice line left on your home number, so callbacks still reach you.
Can I really use both SIMs at the same time abroad?
Yes, and that's the point. A dual SIM phone keeps both lines registered at once and routes each task separately: data over the line you designate, outgoing calls over the default voice line, incoming calls and texts over whichever number the sender used. You give each line a job.
How do I turn on data roaming for just the travel eSIM on an iPhone?
Open Settings, go to Mobile Data, and tap the travel eSIM's line by name rather than the general options. Inside that line sits its own Data Roaming toggle. Switch it on, then open your home line and confirm its Data Roaming is off. The setting appears twice because it belongs to each line.
How do I switch between SIMs on a dual SIM phone?
Depends what you're switching. For data, change which line is selected on the mobile data screen. For outgoing calls, change the default voice line, or set a preferred line per contact. For incoming calls and texts there is nothing to switch: they arrive on the number that was dialed.
Do I need to keep my home SIM turned on, and will it cost me?
Keep it on if you need texts sent to your home number, which for most people means bank codes. With its data roaming off, it stays registered for calls and SMS without opening a data connection. Whether a received text or an answered call gets billed depends on your carrier's roaming terms.
Will my bank's 2FA text messages still arrive?
They arrive as long as your home line is switched on and can register on a network there. Codes are addressed to a number, not to a data connection, so a line with roaming data off still receives them. The failure cases are a country where your carrier has no partner, though Wi-Fi Calling may still cover you there, and a home line you deleted.
Why do my friends say my messages come from a different address?
Check Start New Conversations From in your Messages settings. If it has flipped to your Apple Account email, every new thread shows that address instead of your number, so contacts without it saved see an unknown sender. If the number isn't offered at all, your home line was de-registered.
Is there a way to put just one line in airplane mode?
No. Airplane mode is all or nothing on both iPhone and Android, and it's a standing complaint among frequent travelers. The closest equivalent is turning that one line off with its own toggle, which stops it registering, stops it draining battery hunting for signal, and stops it receiving anything.
The bottom line
Decide by what your home number is for on this trip. If it carries bank codes or people who need to reach you, keep it on with data roaming off, put the travel eSIM on data with its roaming on, and turn Allow Cellular Data Switching off before you leave the airport.
If your carrier has no partner network where you're going, or every code you need already lives in an authenticator app, turn the home line off and run on the travel eSIM alone. And if your existing plan genuinely includes the destination at no extra charge, skip all of this and keep one number. The setup only earns its place when it saves you something.
