To stay connected without a break, keep two things apart: using a travel SIM to get online, and turning on international roaming for the Vietnamese SIM you use every day.

Roaming keeps your home SIM on the network abroad. This matters most when banking apps, e-wallets or social media send an OTP code to your Vietnamese number to verify you. If you do not turn on roaming before you fly, the SIM loses all signal and you cannot receive important messages when you need them.

Things to know about international roaming

1. 1
   Only receive messages (SMS): Most Vietnamese carriers let you receive SMS for free abroad, so keep your home SIM just for notifications and OTP codes.
2. 2
   Avoid calls and sending texts: Unless it is really necessary, do not answer calls, make calls or send texts from your Vietnamese SIM, because international roaming charges are very high.
3. 3
   Turn off Cellular Data on your Vietnamese SIM: Make sure only the travel SIM has data and Data Roaming turned on, so your Vietnamese SIM does not go online by accident and run up charges.
4. 4
   Sign up before you leave: Turn on roaming while you are still in Vietnam; it is quicker and easier, and the activation text is free.

Common ways to turn on international roaming

1. By SMS: Text the free activation code from Vietnam, depending on your carrier:CVQT to 138(Viettel),DK CVQT to 9123(VinaPhone), orDK CVQT to 999(MobiFone).

   

2. In the official app: Log in to My Viettel, My VNPT or My MobiFone, find "International roaming" (Chuyển vùng quốc tế) and switch it on.
3. By USSD code or hotline: Dial*138*1#(Viettel) to do it quickly from the keypad, or call your carrier's customer service to have it turned on before your flight.

See also: How to pay in China

An eSIM (Embedded SIM) is a digital SIM built into the phone's board, so you can activate a plan directly without swapping a physical SIM card. To use one, your phone must be an international (unlocked) model that supports eSIM; this is common on iPhone XS/XR and newer, recent Samsung Galaxy S/Z models and Google Pixel.

• Pros: You get the QR code by email right after buying, there is no SIM tool to lose or home SIM to misplace, and your Vietnamese SIM stays in for OTP codes.
• Cons: It does not work on carrier-locked phones, older phones, or some iPhone models sold in mainland China / Hong Kong (which have 2 physical SIM slots).

How to buy an eSIM on Trip.com

1. 1
   Open the Trip.com app or website, go to eSIM, then choose Mainland China as the destination.

   

2. 2
   You will see a list of many plans. Pick one with the most purchases that also comes with firewall bypass (VPN), so international apps like TikTok and AI tools work normally. If you do not want to spend time searching, you can go straight tothe travel eSIM I used, which has passed 2.2 million sales on Trip.com.

   

3. 3
   Choose a plan for the actual number of days of your trip and the data you want (per day or a total). For example, I picked 100GB/day for an 8-day trip to Chongqing, used it freely, and it cost only about 300,000 VND (about 76 yuan).

   

4. 4
   Tap Đặt (Book) and pay: Enter the email to receive the details, pay, and check your inbox for the QR code to activate it.

   

How to install and use the eSIM

Installing the eSIM before you fly lets you do it calmly on stable Wi-Fi and avoid glitches. But only turn on Data Roaming to activate it once you land in China, so the plan does not start counting days early or quietly use data in Vietnam.

1. Install before the flight (in Vietnam, on Wi-Fi)

1. Step 1: Open the Trip.com app, go to Trips (or Account) > All bookings > choose the eSIM you bought.

   

2. Step 2: Choose one of the 2 ways to install:
3. Option 1 (fastest): Tap "Install with one tap on this phone" > Install > Activate > Continue > choose Abroad > Continue > choose Data Only > Continue > tap Done when you see the completion message.

   

4. Option 2: Tap "View voucher details" to get the QR code and scan it from another device (or tap the "Tap here to install directly" link), then follow the same Activate steps as Option 1.

   

2. Activate as soon as you land in China

1. Go to Settings > Cellular on your phone, choose the travel eSIM you just bought and turn on Turn On This Line.
2. Turn on Data Roaming on the travel eSIM to get online.

   

3. Keep your Vietnamese SIM on but turn off Data Roaming on it, so it only receives OTP codes and SMS for free.
4. If you use 2 SIMs, turn off the number you do not need > Continue. That is it, your eSIM is activated.

   

Handy tip: A newer phone can store and activate several eSIMs at once. Some hotels booked on Trip.com now come with a free eSIM (usually around 3GB/day). You can use these free eSIMs one after another throughout the trip without paying for a new one.

A physical SIM is the classic plastic SIM card with a chip, inserted straight into the phone's SIM tray.

Who should use a physical SIM?

1. People with older phones, carrier-locked phones or devices that do not support eSIM.
2. People using iPhone models made for mainland China, Hong Kong or Macau (2 physical SIM slots, no eSIM).
3. Anyone not comfortable with technical setup on their phone who prefers the classic way: just swap the SIM card and it works.

Pros

1. 1
   Works with almost any device: Runs on most phones with a physical SIM tray (international versions) without complicated setup.
2. 2
   Easy to move around: You can swap it between devices or put it in a pocket Wi-Fi to share the connection with the whole group.
3. 3
   Simple to use: No QR-scanning errors to worry about; when you arrive, just insert the SIM and turn on Data Roaming to get online.

Cons

1. 1
   You have to wait for delivery: It takes time to receive the SIM before your flight, so you cannot buy it at the last minute like an eSIM.
2. 2
   Awkward on single-SIM phones: If your phone has only 1 physical SIM slot, you have to take out your main Vietnamese SIM, so you cannot receive texts or bank OTP codes during the whole trip.
3. 3
   Risk of losing things: It is easy to lose or drop the SIM tool and your Vietnamese SIM while swapping at the airport.