If you are still choosing a provider, start with the guide to Wi-Fi and internet in Vietnam and the Vinaphone tourist SIM guide. The troubleshooting below assumes the SIM is already in your hand.
What "not working" actually means in Vietnam
Travelers use one phrase for four different failures, and the fix is different for each. Misdiagnosing the problem is why people waste an hour at the airport counter.
- No signal at all. The phone shows no bars or "No Service". This points to a locked phone, an unregistered SIM, or a seating problem.
- Signal but no internet. Full bars, apps will not load. Almost always the APN, mobile data, or the wrong line selected in a dual-SIM setup.
- Internet but no calls or SMS. Data works, calls fail. Usually a plan that includes data only, or a voice credit balance of zero.
- SIM not recognised. The phone says "No SIM" or "Invalid SIM". A seating issue, a faulty card, or a carrier-locked device.
Vietnamese carriers (Viettel, Vinaphone, Mobifone, Vietnamobile) operate on GSM, WCDMA and LTE bands that most modern unlocked phones support, so band incompatibility is rare. It is not impossible, particularly with older or grey-market handsets.
Before anything else: is your phone carrier-unlocked?
This is the single most common cause of a foreign SIM failing, and it is the one thing you cannot fix in Vietnam.
On iPhone, go to Settings > General > About and scroll to Carrier Lock. It should say "No SIM restrictions". If it names a carrier, the phone is locked.
On Android, check Settings > Connections > SIM card status, or simply test with a second SIM from a different carrier. If a SIM from another network also fails, the phone is locked.
A phone bought on a carrier payment plan in the US, Japan or Korea is often locked. Unlocking must be done by the original carrier before departure, not in Vietnam. Many travelers only discover this at the airport counter, with the SIM already paid for.
If you are still paying off a phone, assume it is locked until you have checked. A locked phone will show signal bars in some cases but refuse to register on a Vietnamese network.
Step 1: Confirm the SIM is registered to your passport
Vietnam requires identity registration for prepaid SIMs, and an unregistered or incorrectly registered SIM can be cut off. Registration is normally handled by the shop or airport counter at the point of sale, and the passport details must match the person using the SIM.
Ask the seller directly: "Is this SIM registered to my passport, and is it active now?" Get a clear yes or no, not a nod.
Keep the shop's contact details and any receipt. A SIM bought informally, from a street stall or second-hand, may be registered to someone else and stop working without warning. If that happens, no amount of troubleshooting on your phone will help.
The registration step is the one travelers skip. It is also the leading reason a Vietnamese SIM dies after two or three days of working perfectly.
Step 2: Restart, toggle airplane mode, reseat the SIM
The universal first fix, and it works more often than it should.
- Turn airplane mode on for 30 seconds, then off. This forces the phone to re-register on the network.
- Restart the phone.
- For physical SIMs, power off, eject the tray with the tool or a paperclip, check the card is seated in the correct orientation, reinsert, power on.
A SIM that worked yesterday and stopped after a restart is often a seating issue or a temporary network registration glitch. On iPhone, check Settings > Cellular to confirm the line appears at all. If the line is missing entirely, the phone is not reading the card.
Step 3: Check mobile data and data roaming settings
On a Vietnamese prepaid SIM, mobile data is usually off by default, and data roaming must sometimes be enabled even for a domestic SIM in some phone configurations.
- iPhone: Settings > Cellular > Cellular Data Options > Data Roaming
- Android: Settings > Network & internet > SIMs > Roaming
Leaving roaming on with a home SIM still inserted is the fastest way to burn through a home plan. Travelers using dual SIM should disable data on the home line entirely while in Vietnam.
Step 4: Fix the APN settings
An APN (Access Point Name) is the address your phone uses to connect to the carrier's mobile data gateway. A wrong APN produces full bars with no data, which is the most confusing failure mode of all.
| Carrier | APN name | Where to confirm |
|---|---|---|
| Viettel | v-internet | Carrier hotline, SIM packaging, or the shop |
| Vinaphone | m3-world | Carrier hotline, SIM packaging, or the shop |
| Mobifone | m-wap | Carrier hotline, SIM packaging, or the shop |
Confirm these with the carrier or the shop that sold the SIM, because APN values can change.
- iPhone: Settings > Cellular > Cellular Data Network
- Android: Settings > Network & internet > SIMs > Access Point Names
Some phones hide the APN menu until a SIM is inserted. If you cannot find it, insert the SIM first, then look again.
Step 5: Dual-SIM line selection on iPhone and Android
For travelers keeping a home SIM active for calls or banking, this is the most common self-inflicted problem.
On iPhone, go to Settings > Cellular > Cellular Data and choose which line handles data. Then set Default Voice Line and the iMessage & FaceTime line separately.
On Android, go to Settings > Network & internet > SIMs and set the Preferred SIM for data, calls and SMS.
The classic trap: data is set to the home SIM, so the Vietnamese SIM shows no internet even though it is working perfectly. Check this before you touch the APN.
Step 6: eSIM QR code problems
If you bought an eSIM online or at a counter, the failure modes are different.
- QR code already used. Most eSIM QR codes are single-use. If you scanned it once and deleted the profile, you need a new code from the provider.
- QR code scanned but profile not installed. Check Settings > Cellular on iPhone for a pending profile.
- No Wi-Fi during installation. The eSIM profile must be downloaded over a working connection. Install before you fly, on reliable Wi-Fi.
- eSIM limit reached. Most phones cap the number of stored eSIM profiles. Delete an old one first.
iPhone 14 and newer sold in the US have no physical SIM tray, so an eSIM is the only option. Keep the QR code email accessible offline.
Step 7: Top-up and data package activation
A Vietnamese prepaid SIM often has voice credit but no data until a data package is activated. There are two activation routes: an SMS to the carrier's short code, or a USSD code typed into the dialler.
The exact codes vary by carrier and package, and they must be confirmed with the carrier or the shop. The shop receipt or SIM packaging usually lists them.
Top-up cards are sold at convenience stores including Circle K, WinMart and GS25. The top-up code is entered by USSD, not SMS. If the code fails, check that you typed it into the dialler and not into a message.
Symptom, likely cause and fix
| Symptom | Likely cause | First fix | If that fails |
|---|---|---|---|
| No signal at all | Carrier-locked phone or unregistered SIM | Check Carrier Lock; confirm passport registration | Return to the shop with passport and receipt |
| Signal but no internet | Wrong APN, data off, or wrong line selected | Check APN, enable mobile data, verify data line | Restart and re-check APN |
| Internet but no calls | Zero voice credit or data-only plan | Top up and activate a voice package | Ask the shop which plan includes calls |
| SIM not recognised | Seating issue or faulty card | Power off, reseat, restart | Request a replacement SIM |
| Worked, then stopped | Registration issue or plan expired | Check balance and package validity | Return to the point of sale |
When to go back to the store or call the carrier
Set a clear threshold. If the phone is unlocked, the SIM is registered, data and roaming are on, the APN is correct, and there is still no connection after a restart, the problem is on the carrier side.
Return to the point of sale with your passport, the SIM, the phone and the receipt. Vietnamese carrier support by phone is limited in English, so the shop or the hotel front desk is usually the fastest route. A replacement SIM is normally cheap or free if the original is faulty.
Android-specific fixes worth knowing
- Preferred network type set to 5G only. Switch to LTE or "5G/LTE/3G auto" if the local network is LTE.
- Hidden APN menu. Some Android skins hide it until a SIM is inserted.
- Per-app data restrictions. Check Settings > Apps > [app] > Mobile data.
- Battery saver killing background data. Disable it while troubleshooting.
- Samsung and Xiaomi quirk. A second SIM is disabled by default after insertion. Enable it manually in Settings > Connections > SIM card manager.
How to avoid all of this before you fly
- Confirm the phone is carrier-unlocked.
- Decide physical SIM versus eSIM.
- Buy at an official carrier counter or a reputable shop.
- Ask for passport registration on the spot.
- Save the shop's phone number.
- Screenshot the APN and package activation codes.
- Test data before leaving the counter.
If you are arriving by air, the airport arrival guide for 2026 covers where the carrier counters are located. For connectivity alternatives, see the Wi-Fi and internet guide.
FAQ
Q: Why is my SIM card not activating my iPhone? On a Vietnamese SIM, the usual cause is that the SIM was never registered to your passport, or the phone is carrier-locked. Check Settings > General > About > Carrier Lock first, then confirm registration with the shop that sold it.
Q: Why is my iPhone not recognising my new SIM card? Most often a seating issue or a locked phone. Power off, reseat the SIM, restart, and check that the line appears in Settings > Cellular. If it still does not appear, the SIM may be faulty and should be replaced at the point of sale.
Q: How do I force an iPhone to activate a new SIM? Toggle airplane mode on and off, restart the phone, and confirm mobile data and data roaming are enabled for the correct line. If the SIM is registered and the phone is unlocked, activation is usually automatic within a minute or two.
Q: Why is my SIM card not working after transferring it to a new phone? The new phone may be carrier-locked, or the SIM may need reseating. On dual-SIM phones, check that the Vietnamese SIM is selected for mobile data, not the home SIM.
Q: Do I need to register my Vietnamese SIM card with my passport? Yes. Vietnam requires identity registration for prepaid SIMs. This is normally done by the seller at the point of sale. Keep the receipt and the shop's contact details in case the SIM stops working.
Q: What APN should I use for a Vietnamese SIM? Commonly v-internet for Viettel, m3-world for Vinaphone and m-wap for Mobifone, but these can change, so confirm with the carrier or the shop that sold the SIM.
Q: My Vietnamese SIM has signal but no internet. What is wrong? Almost always the APN, mobile data being off, or data assigned to the wrong SIM in a dual-SIM setup. Check all three in that order.
Q: When should I go back to the shop instead of troubleshooting? If the phone is unlocked, the SIM is registered, data and roaming are on, the APN is correct and a restart has not helped, the issue is on the carrier side. Return to the point of sale with your passport, the SIM, the phone and the receipt.