How should I set up dual SIM?

Keep your home SIM for calls and SMS, set the eSIM as the data line, turn off data roaming on the home line, turn it on for the eSIM, and disable automatic data switching. Labelling the lines — 'Home number' and 'Trip data' — makes mistakes far less likely.

This question usually comes up right after finishing an eSIM installation, turning the phone back on, and finding two lines active at once with no clear sense of which one should be doing what. Some people learn the hard way through trial and error — a verification SMS that never arrives, data that refuses to move despite having bought a plan, or a home-SIM roaming charge appearing because it was left switched on without meaning to.

This page walks through correct dual-SIM setup point by point: what the primary line and the data line actually mean, switching roaming on and off for each line, the mistakes travellers make most often, and how to confirm before leaving home that everything is genuinely set up right.

Why getting dual-SIM setup right actually matters

A device that supports an eSIM alongside a physical SIM allows both lines to be active at the same time, but running two lines at once does not mean the phone always picks the right one on its own. Without specifying which line handles calls/SMS and which handles data, some devices switch back and forth unprompted, or stay stuck on a line that was never meant to carry that traffic — burning time troubleshooting mid-trip on something preventable before departure.

Getting the setup right from the start also guards against an unintended charge, particularly home-SIM data roaming that many people forget to switch off, only to find on the bill after returning to Thailand that roaming data was used the whole trip despite intending to rely on the eSIM alone.

What 'primary line' and 'data line' actually mean in settings

A device that supports two lines typically has three separate options in settings: the line used for outgoing calls, the line used for SMS, and the line used for data. These three do not have to be the same line — the home line can be set to handle every call and SMS while the eSIM is set as the only line carrying data, and the system manages each independently according to what has been set.

Most of the confusion comes from assuming there is one line that 'does everything' already, when each of the three actually has to be chosen individually. Without going in and explicitly setting the data option, the phone may simply keep using the home line for data the way it did before the eSIM was installed — one of the leading reasons someone buys an eSIM and still ends up throttled or still burning through the home line's roaming data anyway.

Setting the home SIM to handle calls and SMS

The home SIM should be set as the line that makes outgoing calls and receives SMS, because the Thai number is what people call back and what accounts such as a bank or verification app are tied to. Switching SMS over to the eSIM line instead means an OTP tied to the Thai number never arrives at all, since a trip eSIM usually has no number linked to those accounts in the first place.

This is worth checking immediately after finishing the eSIM install, since some devices default the line order based on install sequence rather than what the user intended. Going back into the line settings page after installing to confirm calls/SMS are still tied to the home SIM is a step not worth skipping.

Setting the eSIM as the data line

Once calls/SMS are confirmed still on the home SIM, the next step is going into data line settings and switching it to the eSIM instead of the home SIM, so all internet traffic runs directly through the plan bought for that trip rather than through the home SIM's roaming data, which is usually a good deal more expensive.

Most devices have a direct data-line option in the line/SIM settings page. Some call it 'mobile data line', others 'cellular data' — the label differs by operating system, but the meaning is the same: choosing which line internet traffic runs through.

Turning off data roaming on the home SIM

Even after setting the data line to the eSIM, leaving data roaming switched on for the home SIM still risks unintended roaming data use — for instance, if the eSIM's signal weakens momentarily, some devices switch to whichever line has stronger signal to keep the connection alive. If the home SIM still has roaming switched on, there is a chance of quietly slipping onto the home SIM's roaming data without noticing.

Switching off the home SIM's data roaming cleanly before departure is therefore an important safeguard: even if the phone tries to switch lines, there is no roaming data available to switch onto in the first place.

Turning on data roaming for the eSIM

Most travel eSIM plans work through a partner network in the destination country, which from the device's point of view technically counts as roaming. If data roaming is switched off on the eSIM line, the phone refuses to connect to the internet even when the profile has been installed and activated correctly. Data roaming on the eSIM line needs to stay switched on — the opposite of what applies to the home SIM.

This is a common misunderstanding, since 'roaming' tends to be remembered as something to switch off to save money, when the rule differs by line. The home SIM turns roaming off to avoid a charge; the eSIM turns roaming on because that is simply the normal mechanism of the plan that was bought.

Disabling automatic data line switching

Many devices carry a feature that automatically switches data over to the other line whenever the line currently set has weak signal or is briefly unusable. That feature is useful in some situations, but during a trip where control over exactly which line carries data matters, it can switch the phone onto the home SIM's roaming data without meaning to.

Switching this automatic behaviour off before departure helps make sure data runs through the eSIM only, as intended, even when the eSIM's signal dips briefly in a particular area.

Labelling each line clearly

Most devices allow renaming each line in settings rather than displaying the operator's own name, which is easy to mix up in a hurry. Giving each line a clearly different label — 'Home number' for the home SIM and 'Trip data' for the eSIM — cuts down on picking the wrong line when switching one on or off mid-trip.

Another benefit of labelling is that when a notification or a signal icon appears on the screen, it is immediately clear which line is active, without needing to open the settings page to double-check every time.

Which line verification and chat apps should stay tied to

Apps tied to a phone number, such as a banking app or anything using the Thai number to verify via SMS, should always be left running through the home SIM, since there is no practical way to switch that verification to a different number anyway. Chat apps that rely mainly on the internet, such as data-based messaging apps, work over whichever line has an active data connection, regardless of which one that is.

If unsure which apps are directly tied to a phone number, a simple check is recalling whether signing up for that app the first time required entering a phone number and waiting for an SMS confirmation. If so, that app is tied to the number and should be left running through the home SIM.

Menu positions differ by manufacturer and operating system version

The settings pages for primary line/data line, roaming on and off, and automatic switching sit in different places depending on device manufacturer and operating system version. Some devices bundle everything into a single section called SIM management; others split it across several sub-pages by topic. This page cannot give one menu path that matches every model.

If it cannot be found where expected, typing data line, roaming, or SIM management into the settings app's own search box usually reaches the right menu faster than hunting through it manually. The plan's own install guide often carries illustrated steps that help too.

Testing the setup before leaving home

Once everything has been set, it is worth testing that the home SIM genuinely places a call, genuinely receives a test SMS, and that a web page loads over the eSIM's data before leaving home. Even though the eSIM often does not actually activate until reaching the destination country or the plan's start date, checking that the profile is installed and the line settings are correct can still be done from home.

If the plan allows use immediately after install, actually testing a web page load is the clearest confirmation that the data setting genuinely points to the eSIM, before having to rely on that setting for real mid-trip.

1. 1Set calls/SMS to the home SIMOpen line settings and choose the home SIM for outgoing calls and SMS.
2. 2Set the data line to the eSIMChoose the eSIM as the line that carries all internet traffic.
3. 3Turn off data roaming on the home SIMPrevents unintentionally slipping onto the home SIM's roaming data.
4. 4Turn on data roaming for the eSIMThe trip plan works through a partner network, so roaming needs to stay on.
5. 5Turn off automatic data line switchingPrevents the phone switching to the home SIM on its own when eSIM signal briefly weakens.
6. 6Label each line clearly and testLabel them, e.g. 'Home number' and 'Trip data', then test a call and a web page load.

Common mistakes made with dual-SIM setup

The first mistake is forgetting to switch off the home SIM's data roaming, leaving a line ready to burn roaming data at any moment even when the intention was to rely on the eSIM alone. The second is switching off the eSIM's own data roaming out of a mistaken belief that roaming should always be off, when the eSIM needs it switched on by the nature of the plan.

The third is not explicitly setting the SMS line, so an OTP sometimes tries routing through the eSIM, which has no number attached to it. The fourth is skipping the pre-departure test, only to run into the problem after arriving at the destination, where fixing it is a good deal harder with no Wi-Fi to fall back on.

If calls/SMS stop arriving after finishing setup

If a call cannot go through or an SMS stops arriving after setup, go back into line settings to confirm calls/SMS are still tied to the home SIM. Some devices reset this back to a default after a restart or an operating system update, so it is worth re-checking after either of those.

If the setting is confirmed correct but the problem persists, the cause may be the home SIM's own signal in that area, which is a matter of the home operator's network and not something this page can confirm on their behalf.

If data does not work on the eSIM line

If a web page will not load despite the data line being set to the eSIM, the first thing to check is whether the eSIM's data roaming has genuinely been switched on, since that is the most common cause. The second is confirming the eSIM profile has actually been activated rather than still sitting as installed-but-not-activated.

If both of those check out but there is still no data, the cause may be the partner network's signal in that particular area, which depends on the operator and destination country. It is worth contacting the team with the device model, the country, and a screenshot of the settings already checked, so the follow-up can move faster.

• Home SIM set to handle calls/SMS only
• eSIM set as the sole primary data line
• Data roaming off on the home SIM
• Data roaming on for the eSIM
• Automatic data line switching disabled
• Lines clearly labelled and tested before departure

See the full eSIM install walkthrough

Check whether your device supports eSIM