What an eSIM actually is on an iPhone
An eSIM is a digital SIM built into the phone. Instead of a plastic card, you scan a QR code or use an activation link, and the carrier profile is written to the device. On iPhone you manage it in Settings > Mobile Service (called Cellular in US English), where each plan appears as a separate line you can label — for example "Home" and "Vietnam".
Because it is a separate line, the Vietnam eSIM does not replace your home SIM. That is the part most travellers care about: your normal number stays in the phone.
Which iPhones support eSIM
Apple maintains the definitive list, and it is worth checking your exact model rather than trusting a shop page. As a rule of thumb:
| iPhone model | eSIM support |
|---|---|
| iPhone XS, XS Max, XR | Yes |
| iPhone 11 series and later | Yes |
| iPhone SE (2nd generation, 2020) and later | Yes |
| iPhone X and earlier | No |
| iPhone models sold in mainland China (most) | Physical dual-SIM, no eSIM |
Two exceptions trip people up. First, iPhones bought in mainland China, Hong Kong and Macao often have two physical SIM slots and no eSIM hardware. Second, a carrier-locked iPhone may refuse a plan from another provider until it is unlocked. Check Settings > General > About for a "Carrier Lock" line, and confirm your model on the VietSIM phone compatibility page before you buy.
Dual-SIM on iPhone: how it behaves in Vietnam
Once both lines are active you choose, per line, what it is used for:
- Mobile Data — set this to the Vietnam eSIM.
- Default Voice Line — this is the line that dials out and sends SMS. Keep it as your home line if you want to receive bank codes, or switch it if you buy a local Vietnamese number.
- Allow Mobile Data Switching — leave this off. It lets the phone fall back to your home line for data, which can trigger roaming charges.
- Data Roaming — turn this on for the Vietnam eSIM line only. eSIM data plans typically need roaming enabled to connect to a local network.
A checklist for the flight:
- Install the Vietnam eSIM at home, on Wi-Fi, before departure.
- Label the lines so you can tell them apart.
- Set Mobile Data to the Vietnam eSIM.
- Turn Data Roaming on for the Vietnam eSIM, off for your home line.
- Turn off Mobile Data Switching.
- Leave your home SIM active if you want calls and texts.
If you would rather keep your home line completely silent, you can turn that line off in Settings and re-enable it when you land back home.
Will WhatsApp still work?
Yes. WhatsApp is tied to your phone number for registration, but messages, calls and media travel over whatever data connection the phone is using. With Mobile Data pointed at the Vietnam eSIM, WhatsApp works normally and your contacts still see your usual number.
A few practical notes:
- Do not delete and reinstall WhatsApp while travelling unless you can receive an SMS on your registered number. Re-registration needs that code.
- Keep your home SIM active if your WhatsApp number is on it and you want to be reachable on that number. An inactive line can still receive some messages, but behaviour varies by carrier.
- iMessage and FaceTime work the same way — they follow your Apple ID and the data connection.
- Wi-Fi calling on your home line may or may not be supported by your carrier abroad; that is a carrier setting, not something the eSIM controls.
If you use a Vietnamese number for local calls, remember that most travellers will still reach you on WhatsApp rather than a local number.
Choosing between a Vietnam eSIM and a local SIM
| Vietnam eSIM | Local physical SIM | |
|---|---|---|
| Buy before you fly | Yes | No |
| Keeps your home SIM in the phone | Yes | Only if you have a second slot |
| Registration paperwork | Handled at purchase | Passport registration in shop |
| Best for | Short trips, multiple countries, keeping your number | Long stays, heavy local calling |
If your trip is a week or two and you mainly need maps, messaging and bookings, the eSIM route is the lower-friction option. If you are staying for months and want a Vietnamese number for calls, a local SIM still makes sense.
Common iPhone eSIM problems
- "No Service" after landing. Check that Data Roaming is on for the eSIM line and that Mobile Data is pointed at it.
- QR code will not scan. You can often add the plan manually using the SM-DP+ address and activation code from your order.
- Two lines, no data. Turn the phone off and on once; the phone needs to re-register both lines on the local network.
- Plan installed but not active. Some eSIMs start their validity on first connection to the network, so install early and activate on arrival.
Step-by-step instructions for each of these are in the VietSIM help centre, including installation and activation guides.
Before you buy
Match the plan to your trip length and data habits rather than buying the biggest bundle. A traveller using maps, messaging and a few bookings will use far less data than someone streaming video or working remotely. Check the product page for the network, validity period and activation method — those details come from the plan record, not from general assumptions about Vietnam.
If you have already ordered and want to check status, use the order lookup. For Vietnamese readers comparing domestic mobile plans, the plan catalogue lists packages exactly as they are sold, for example V90B.