Keeping your own number abroad without getting a €70 bill
How dual SIM actually splits calls, texts and data — what data roaming off does and does not stop, and where Wi-Fi calling fits.
onesim team
The fear that keeps people on expensive roaming is a simple one: if I use a travel eSIM, does my own number stop working?
It does not. But the way a dual-SIM phone splits things is worth understanding properly, because the one setting everyone toggles does less than they think.
What "data roaming off" actually does
Turning data roaming off on a line stops data on that line. That is all it stops.
The line stays registered on a foreign network, which means:
- Incoming calls still arrive. Depending on your tariff, receiving a call abroad may be charged — in many countries outside your free zone it is.
- Incoming texts still arrive. Receiving SMS is free on almost every tariff, everywhere. This matters, because it is how your bank sends a code.
- Outgoing calls and texts still work, at roaming rates, if you dial them.
So the standard traveller arrangement is: home SIM on, its data roaming off; travel eSIM on, its data roaming on, selected as the mobile data line.
You keep your number. You keep your texts. You do not pay for data.
Which line does what
On a dual-SIM phone, each of these is a separate choice, and they are separate settings:
| What | Setting | Sensible choice abroad |
|---|---|---|
Mobile data | One line only | The travel eSIM |
Outgoing calls | Default line, or ask each time | Your home line, or "Ask" |
iMessage and FaceTime | Per-line selection | Your home number |
SMS | Per-line, usually follows the default | Your home line |
On iPhone, all four live under Settings → Mobile Data. On Android they are under Settings → Network & internet → SIMs, with a "preferred SIM" for calls, texts and data.
Setting outgoing calls to Ask is the safe default: the phone asks you which line to use each time, so you never place an accidental roaming call.
Wi-Fi calling: useful, and not universal
Wi-Fi calling routes your calls and texts over an internet connection while behaving as though you were at home — so calling a domestic number from a hotel in Bangkok costs what it would cost from your sofa.
Two caveats. First, not every carrier allows it abroad; some disable it outside the home country entirely, and a few disable it in specific countries. Second, it has to be turned on before you leave, and on some networks you need a registered emergency address on file for it to activate.
Test it before you fly: turn on Wi-Fi calling, put the phone in airplane mode with Wi-Fi on, and try a call. If it connects at home, it will work abroad wherever the carrier permits it.
Two-factor codes, the real problem
The thing that catches travellers is not calls. It is a bank that sends a one-time code by SMS to a number that is sitting at home in a drawer.
Before you fly, for anything with a login
- Keep your home SIM in the phone — receiving SMS abroad is free almost everywhere
- Move what you can from SMS codes to an authenticator app, which works with no signal at all
- Save backup codes somewhere you can reach without the phone
- Check whether your bank needs an app confirmation instead of a code, and that the app works over data
- If you use an eSIM-only phone with a single line, do not delete your home profile to make room
Authenticator apps — Google Authenticator, Authy, 1Password, the built-in code generators in iOS and Android — generate codes from a clock, not from a network. They work in airplane mode. Moving your important accounts to one before a long trip is the single most useful hour of preparation there is.
If you want to go further
- Call forwarding sends your number to a local number or to voicemail. Your home carrier usually charges for the forwarded leg, so it is cheaper for occasional calls than for a talkative month.
- A second eSIM with a local number is the answer if you genuinely need a local number — for a delivery, a landlord or a local bank. Some travel providers sell them; most data-only travel eSIMs, including ours, do not.
- Number parking with a cheap domestic plan is what long-term travellers do: keep the number alive for two-factor codes on the cheapest possible tariff and use data elsewhere.
The shape of it
Home SIM in, data roaming off, keeps your number and your texts. Travel eSIM on with data roaming on, carries everything that matters. Calls happen over WhatsApp or FaceTime, as they already do.
That arrangement costs nothing beyond the package and it takes four settings to set up.
