Your phone is the key to receiving OTPs, opening electronic tickets, finding addresses, and calling family. But buying data is not enough: your SIM may not have roaming enabled, an eSIM may be incompatible, your phone may be carrier-locked, and maps and backup codes should be downloaded while you still have familiar Wi-Fi.
Choose a connection and check your carrier
You can keep your Vietnamese SIM for SMS, sign up for roaming, use a travel eSIM for data, or buy a local SIM. Each option has different requirements for the device, subscriber verification, coverage, and fees; an advertised “unlimited” plan is not a guarantee for every city.
Before buying, open your carrier’s official site to check China coverage, partner networks, voice/SMS/data, how to enable roaming, limits, and support channels. See Viettel international roaming, VinaPhone international roaming, and MobiFone international roaming. Screenshot the plan name, validity time, and lookup instructions. Terms can change by subscriber; MobiFone’s Go China plan also currently notes VoLTE and network coverage, so ask the carrier directly.
Check SIM and eSIM compatibility
Note your phone model, sales region, operating-system version, and number of SIM slots. Check eSIM support, dual-line capability, and carrier lock status. For iPhone, Apple explains how to open Settings → General → About → Carrier Lock; an unlocked phone shows “No SIM restrictions.” Apple also says to check the carrier, region, and supported bands.
On Android, menus differ by manufacturer. Google explains that a phone must be SIM-unlocked to use another carrier and that 4G/5G support should be checked with the carrier. If you cannot find “Add eSIM,” ask the manufacturer before buying a QR code. A data-only eSIM may not provide a number for SMS or calls; keep your Vietnamese SIM in the physical slot, or use a second phone or backup device if OTPs matter. Ask about SMS and call charges and when data roaming needs to be enabled.
Install an eSIM with the validity clock in mind
Do not install a China-only eSIM early out of habit. First read the provider’s terms: does the plan count days from installation, activation, or the first time it connects to the destination network? Some plans start immediately upon activation. If you need to install it ahead of time, do so only after understanding the validity period and refund conditions. Apple notes that an iPhone needs carrier eSIM support and usually Wi-Fi or a hotspot for activation.
While still in Vietnam, install it according to the instructions, save the order number, APN, and offline support channel, and treat the QR code like a password. Name the lines “VN - OTP” and “China - data,” choose the correct data line, turn off automatic data switching, and disable roaming on the Vietnamese SIM if you do not need it. A plan limited to China may have no data in Vietnam; that is not necessarily an error. There is no need to send an SMS from a data-only eSIM just to test it. Check its installation status, and while your Vietnamese SIM is active, test SMS and calls according to the carrier’s instructions.
Maps and official apps
Google Maps lets you download an area for offline use, but Google warns that some countries or regions are not supported because of data limitations. Try downloading the exact area for your trip while you still have a connection; if the option is unavailable, save Chinese addresses, booking references, stations, and pickup points as screenshots or in a map app supported in the area. Offline maps also do not include complete live traffic, public-transit, or alternative-route information.
Download apps from the App Store or Google Play and verify the publisher. Travel China is the official tourism portal of China’s Ministry of Culture and Tourism; it provides preparation guidance and introduces Nihao China for international visitors. Open flight, train, hotel, and attraction bookings from official websites or apps; avoid APKs or support links sent privately. On Android, keep Google Play Protect enabled.
Keep OTPs and your device secure
List the accounts that require codes: banking, email, tickets, and work. Check recovery numbers, backup email, and whether international SMS or calls will work. Set up an authenticator app, security key, or backup codes; Google explains that backup codes are single-use and must not be shared.
Never read an OTP to someone claiming to be support. Turn on a screen lock, update the phone, enable Find My or the device-finding feature, back up your data, and turn off automatic connections to unfamiliar Wi-Fi. A VPN is also only a tool whose behavior can vary by network and time; it is not a promise of 100% availability or a blanket guarantee that every use is lawful.
Check after landing
- Turn on the correct data line and enable roaming only for the line you selected.
- Check the network name, remaining data, and a simple web page.
- Open your downloaded map or your note with the Chinese address.
- Do not request OTPs repeatedly if you do not need them yet.
- If service is lost, turn on airplane mode for about 30 seconds and try again, or contact the carrier; do not delete the eSIM immediately.
Read the guide to installing the app on each device, the VPN checklist before your trip, and the overall trip-preparation checklist.