When you install a second line (a travel eSIM alongside your physical SIM, or a second eSIM), the operating system doesn't decide on its own which line is "the right one" for each service. Calls, messages and mobile data each have their own separate default line, and by default the phone tends to leave everything configured on whichever line was already active — that is, the one installed first.

The result is that, until you step in manually, the new eSIM stays present and functional on the phone but isn't actually used for data or calls: the device keeps routing everything through the original line, with very concrete consequences if you're abroad and that line doesn't have an affordable data plan for that country.

It affects both, and that's exactly what causes most of the confusion. On both iPhone and Android, the default line for mobile data and the default line for calls and SMS are two distinct settings, which can be assigned to different lines but need to be checked and corrected separately.

It's perfectly normal, and often the ideal setup while traveling, to keep your home SIM as the default for calls (so you stay reachable on your usual number) and the destination eSIM as the default for mobile data. The problem arises when one of these two settings stays "stuck" on the wrong line because it was never changed after installing the new eSIM.

Most eSIM guides focus on the installation step: scanning the QR code, naming the line, confirming activation. This step almost always succeeds without a hitch, and that's exactly why users assume everything works correctly and automatically from that point on.

The problem, however, surfaces later, often once already abroad: data doesn't work as expected, or a roaming data usage notification shows up on the home line. At that point it looks like a network coverage issue or an APN misconfiguration, when in most cases it's simply a default line that was left set to the wrong one.

Go to Settings > Cellular and tap Cellular Data: here you can select which of the two lines (physical SIM or eSIM) the phone should use as the default for browsing, downloading and using apps that need a connection.

After installing a new travel eSIM, this is the first thing to check: explicitly select the newly activated eSIM as the data line, instead of assuming the phone already set it correctly on its own.

Right below the default data line choice, iPhone offers the Allow Cellular Data Switching option. When enabled, the phone can automatically switch to the other line whenever the one chosen as default doesn't have enough signal or data coverage.

It's a feature designed to keep you connected, but while traveling it can backfire: if you haven't turned off roaming on the home line, a moment of weak signal on the destination eSIM can trigger an automatic switch to your home SIM, racking up roaming data usage without you noticing. For predictable use while traveling, many users prefer to turn this automatic switching off.

Still in Settings > Cellular, the Default Voice Line option lets you choose which line the phone automatically uses when you place a call without manually specifying the other line from the Phone app.

While traveling, the most common — and often most convenient — setup is to keep your home SIM as the default voice line, so you stay reachable on your usual number for friends, family and work contacts, and use the destination eSIM exclusively for data.

Yes: iMessage and FaceTime are tied to a specific phone number, not necessarily to the default line for calls. If you notice green SMS messages being sent from the line you didn't expect, it's worth checking the Messages and FaceTime settings separately, in addition to the general Cellular configuration.

It's also worth remembering that with dual SIM on iPhone, you can't have two WhatsApp accounts active at the same time in the same app: if you want to use the travel eSIM for WhatsApp too, the only workable route is WhatsApp Business paired with the second number.

On most Android phones, the general path is Settings > Network & Internet > SIMs (on some models "Mobile network" or "SIM & network"). Inside you'll find three distinct entries, each assignable separately: Calls, SMS and Mobile data.

After installing a new eSIM, open this panel and check each of the three entries one by one: it's very common for mobile data to still be assigned to the first SIM installed, while the new eSIM stays active but unused for the internet connection.

Yes, every manufacturer customizes the interface slightly. On Samsung Galaxy the panel is usually under Settings > Connections > SIM Card Manager; on Google Pixel it's Settings > Network & Internet > SIMs; on Motorola and other near-stock Android phones, Settings > Network & Internet > SIMs & Mobile Network.

The underlying logic, though, is the same everywhere: a dedicated SIM management panel where you assign, line by line, which card to use for calls, SMS and data. If you can't find the exact entry described here, simply search "SIM" in your model's settings search bar.

Many Android phones offer an option (often called "Switch automatically between SIMs" or similar) that lets the phone temporarily use the other SIM for data whenever the preferred one can't connect. It's useful for never losing connectivity, but while traveling it can cause the same unwanted effect seen on iPhone: roaming data usage on the home line without you noticing.

If you want absolute certainty that only the destination eSIM is ever used for data, the safest choice is to turn this automatic switching off, or to disable data roaming on the line you never want to use abroad.

On several Android phones, particularly Samsung and Motorola models, there's a feature that automatically assigns a specific SIM to a single contact after you've called them several times with that line (on Samsung it's called "Smart SIM"). This per-contact assignment takes priority over the general default voice line.

If you notice some calls always go out on the "wrong" line despite having changed the default voice line, check that individual contact's settings in your address book: it was probably assigned a specific SIM manually (or automatically) at some point in the past.

Practical rule: turn on data roaming on the destination eSIM you've chosen as the default data line, otherwise it won't be able to connect to the local network in the country you're in. Turn data roaming off on the home line you don't want to use for browsing, so that even in case of an accidental automatic switch, usage stays blocked.

Voice calls and SMS follow different logic from data, and remain governed by the default voice line you've set, regardless of whether data roaming is on or off on that line.

It's always better to do it beforehand, calmly and with a stable Wi-Fi connection available, rather than right after landing with only the local network signal to rely on. Install the eSIM, immediately assign the data line and, if needed, the voice line, turn off roaming on the line you won't use, and verify with a practical test (a web page, a test call) that everything works as expected.

Renaming the lines with clear labels like "Home", "Travel" or "Work" in the phone's settings makes it much easier to stay oriented later on, whenever you need to double-check or change the configuration.

• Default data line set to the destination eSIM.

• Automatic data switching turned off, if you want the certainty of always using the same line.

• Default voice line set according to your preferences (usually the home line, to stay reachable).

• Data roaming on for the travel eSIM and off for the line you don't want to use.

• Line labels renamed clearly, so you don't get confused later on.