Quick facts
| Home SIM | Travel eSIM | |
|---|---|---|
| Carries mobile data | No, roaming off | Yes |
| Receives your bank’s SMS | Yes | No |
| Calls out by default | Usually yes | Not needed |
| Costs you while abroad | Nothing, if roaming is off | The price you paid |
| Shares the hotspot | No | Yes |
Disclosure: VietSIM sells prepaid eSIMs for Vietnam. This page is the setup, not the sales pitch.
Why both, rather than one
Your home number is worth keeping active for one reason: the SMS code. Banks, airlines and a dozen accounts send them to the number you registered years ago, and no local plan can receive them for you.
Vietnamese data is worth buying for the opposite reason: it is the thing you will use all day, and paying home rates for it is the expensive way round.
Running both puts each half on the side where it costs less. Apple describes exactly this as one of the ordinary uses of Dual SIM: add a local data plan when you travel outside your country.
The three settings that matter
1. Set the eSIM as the data line. On iPhone: Settings, then Cellular or Mobile Data, then Cellular Data, then pick the travel line. Apple is clear that only one line carries data at a time, so this choice is what decides where your money goes.
2. Turn data roaming off on the home line. This is the one that prevents the bill people come home to. With it off, the home line still receives calls and SMS and cannot spend anything on data.
3. Label the lines. Call one “Home” and the other “Vietnam” when the phone asks. Every later menu refers to them by those names, and a mislabelled line is how people end up sending data through the wrong one.
Apple also warns of a side effect worth knowing: if data roaming is on for the line carrying data, the voice-only number can lose Visual Voicemail and MMS.
Android
The names differ by manufacturer, but the settings are the same three: a SIM manager or Dual SIM screen where each line can be named, a “mobile data” preference that picks the data line, and a roaming switch per line. Find them before you fly rather than in an airport, because the wording is never quite what you expect.
Two eSIMs, no card
Apple notes that iPhone 13 models and later support Dual SIM with two eSIMs, not only a physical SIM plus an eSIM. If your home operator can convert your existing number to an eSIM, you can travel with no card in the phone at all, which removes the classic holiday mistake of losing the tiny tray.
Which phones take an eSIM at all is listed in does a Vietnam eSIM come with a phone number, and the case for local data over roaming is made in roaming or a local eSIM. What to buy once the setup is clear is in the Vietnam eSIM store.
Before you fly
- Install the eSIM profile on Wi-Fi at home, then leave it switched off.
- Label both lines.
- Turn data roaming off on the home line.
- Land, switch the eSIM on, and set it as the data line.