Sophie
5 November 2026 · 12 min read
You're off to Morocco, Thailand or Brazil with a local eSIM bought before departure, and your French line still active alongside it. You send a text to your family and everything looks normal. In a dual-SIM phone, a text goes out on the line designated as the "default line" for messaging — and that is almost never the one you just set up for data. Worse: when you reply inside an existing conversation thread, your phone reuses that thread's original SIM, even if it is roaming outside the European Union. A text sent on the wrong line can cost several tens of cents instead of being included in your plan, and your contacts sometimes see a number they don't recognise.
Updated 5 November 2026.
Which line actually sends my text on a dual-SIM phone?
The one set as the default line for texts in your settings — a parameter separate from the one handling calls and the one handling mobile data. All three can point to three different SIMs.
This is the point almost everyone misses. When you insert a local SIM or activate a tourist eSIM, the phone usually offers to use it for mobile data. You accept, browsing works, and you conclude that "everything now goes through the new SIM". Wrong: messaging stays on the old default line, meaning your French number in roaming.
On Android (Google Messages, 2026 version), the path is: Settings → Network & internet → SIMs → SIM for SMS. On iPhone (iOS 18 and later): Settings → Apps → Messages → Send from, and for the general logic Settings → Cellular → Cellular Data Default and Default Voice Line.
Three independent settings to check before you leave:
| Setting | What it controls | Recommended choice when travelling |
|---|---|---|
| Mobile data / cellular data | Browsing, apps, RCS, MMS | Local SIM or tourist eSIM |
| Default calling line | Number used to place calls | Depending on your needs (French line for your bank) |
| Default SMS line | Number that sends your texts | The one whose international texts are included |
Until these three boxes are set deliberately, you don't know what your phone is doing with your messages.
Why does my text go out on the French line even though I have a local eSIM?
Because conversation threads are "glued" to a SIM. Every conversation saved in your messaging app remembers the line it started on, and a reply goes back out on that line regardless of the default setting.
In practice: the conversation with your mother has existed for three years on your French number. You land in Bangkok, activate a local eSIM, even set the default SMS line to that eSIM — and when you reply in the existing thread, the message still goes out on the French line, roaming outside the EU. The new setting only applies to new conversations.
To force the sending line in an existing thread:
- Open the conversation concerned.
- On Android, tap the contact's name at the top, then look for the SIM or Switch SIM entry; depending on the manufacturer's skin (Samsung One UI, Pixel), the selector also sits next to the text input field.
- On iPhone, tap the contact's name at the top of the thread: the line in use is shown and can be changed.
- Check before sending: the carrier name or "SIM 1 / SIM 2" should appear near the send button.
- If in doubt, start a new conversation to the same number: it will use your current default line.
Simple rule: if the SIM selector isn't visible on screen when you press "send", you don't know who's paying.
How much does a text sent on the wrong line cost?
It all depends on the zone. Within the European Union, including the European Economic Area, the "Roaming like at home" regulation — extended until 2032 — requires outgoing texts to be deducted from your French plan exactly as they are at home. Outside that zone, sending is outside your plan allowance with virtually every French carrier, and charged per message.
To find the rate that applies to your line, the only reliable source is your carrier's "international and roaming" price list (Orange, SFR, Bouygues Telecom, Free Mobile or your virtual operator), published on its website and updated regularly. In its factsheet on l'itinérance internationale (international roaming), the Arcep points out that non-EU rates are set freely by carriers: there is no European regulatory cap, contrary to what many travellers believe.
Three billing traps specific to dual SIM:
- the "dormant" line costs money anyway: a French line left active while roaming outside the EU may not send any texts, but it does receive them, it registers on the local network, and any reply sent by mistake is billed at international rates;
- an MMS is not a text: a message with a photo switches to MMS, which uses data and is often charged per item outside the EU. If your default data is on the local eSIM but sending happens on the French line, the MMS can fail while still being retried several times;
- billing arrives late: settlements between foreign and French carriers often take several weeks, as we explain in our guide to texting and data outside the European Union.
Does my contact see my French number or the local one?
They see the number of the line that sent the message. If the text goes out on the local eSIM, your contact receives a message from a Thai, Moroccan or Mexican number that isn't in their address book — with all the consequences that implies.
This detail has three very concrete effects:
- your messages look like spam: an unknown foreign number writing "it's me, I've arrived" often ends up ignored, or even reported;
- the conversation splits in two: a new thread appears on your recipient's phone, and the history falls apart;
- verification codes don't follow you: your bank, the tax office or your email provider send their code to the registered number, meaning your French line. If that line is switched off to save money, the code will never arrive.
That's why we rarely advise simply shutting down the French line while travelling. The topic is close to that of texts lost when you use a local SIM: the logic is identical, and dual SIM merely creates the illusion of keeping everything.
Does RCS work on the second SIM?
Rarely in a reliable way. RCS, the successor to SMS backed by Google Messages and by carriers, is activated per line and requires registration with the relevant carrier's network. On a dual-SIM phone, only one line is generally registered for RCS at any given moment.
The practical consequences:
- your usual RCS thread, tied to the French line, may fall back to standard SMS as soon as default data switches to the local eSIM;
- "data only" tourist eSIMs have neither a usable number nor a carrier messaging service: they will never do RCS;
- an RCS → SMS fallback changes your billing without warning: what used to travel over the internet becomes a message charged by your carrier.
If you're counting on rich messaging while travelling, it's better to understand the fallback before you go: we covered it in detail in our comparison of RCS versus SMS. For a one-off message without involving either of your two lines, sending a free text from your browser remains the most neutral option: nothing is deducted from your plan, and you don't touch any settings.
How to set up your phone before departure: the six-step procedure
Ten minutes at the airport beats a bill when you get home. The sequence below works on most dual-SIM phones sold in France, whether you have two physical SIMs, one SIM plus an eSIM, or two eSIMs.
- Name your lines. In the SIM settings, rename them "FR Orange" and "eSIM Thailand" rather than "SIM 1" and "SIM 2". Every selector will then show a readable name, and you'll stop picking the wrong one.
- Choose the data line. Put data on the local eSIM and explicitly disable data roaming on the French line: that's the heaviest expense outside the EU.
- Choose the default SMS line. If your French plan includes texts from the zone you're visiting (as in the EU), keep the French line: your contacts will see your usual number. Outside the EU, switch to the local line for new messages, bearing in mind that the number displayed changes.
- Keep the French line reachable. Turning off data doesn't stop you receiving texts: reception is free for you in the vast majority of cases, which preserves your banking verification codes.
- Open your three most frequent conversations and set the SIM for each one, since existing threads ignore the global setting.
- Send a test message to someone close and ask them which number shows up. That's the only check that never lies.
For step 1, a recent dual-SIM phone makes life much easier: models that combine a physical slot with an eSIM let you keep the French SIM untouched. If you juggle several physical SIMs across several trips, a small SIM card storage case stops you losing the French chip at the bottom of a bag — and a SIM you can't find again can be replaced, but at the cost of a trip to the shop when you get back.
The edge cases that catch people out most
The phone that decides for itself
Some manufacturers offer an "ask every time you send" or "smart SIM" option, which picks the line based on the recipient's country or your call history. Handy in France, risky abroad: an algorithm may decide to use the French line because the number starts with +33 — exactly what you want to avoid outside the EU. When travelling, go for manual selection, or the "ask every time" option.
Both lines roaming at once
If you travel with two French lines (one work, one personal), both register on the local network and each applies its own price list. One plan may include the zone, the other may not. The danger is no longer picking the wrong line through ignorance of the settings, but not knowing which one is covered. Readers who manage two lines day to day will find the blind spots catalogued in our guide to using dual SIM to split your bill.
The disappearing 2G/3G fallback
In several countries, 2G and 3G networks are shutting down, as in France. A cheap local eSIM relying on a limited partner network can lose voice and texting while keeping data. If that's your default SMS line, your messages fail silently. In that situation, a portable solar charger or a high-capacity power bank won't change anything — but it at least keeps the phone alive long enough to switch lines and find a usable network.
Dual SIM in a secondary phone
Many travellers leave with a second dedicated device, often an entry-level dual-SIM phone, to isolate their travel line. The approach is sound, on one condition: check before departure that the device supports the bands used in the country you're visiting. An unlocked 4G/5G-compatible phone covering local bands saves you from discovering on arrival that the eSIM activates but picks up nothing.
Summary
| Situation | What the phone does | What you should do |
|---|---|---|
| Local eSIM activated for data | Texts keep going out on the French line | Set "SIM for SMS" separately |
| Reply in an old thread | Reuses the thread's original SIM | Change the SIM inside the conversation |
| French line switched off | No more verification codes received | Keep the line active, turn off data |
| Text sent on the local line | The contact sees an unknown number | Warn your family and friends before leaving |
| Data-only eSIM | Neither texts nor RCS possible | Use a web solution or the French line |
A dual-SIM phone doesn't simplify travel: it simply doubles the number of decisions to make. Make them before take-off, not after your first unanswered message.
Frequently asked questions
Can I send a text from my French line while using my local eSIM's data?
Yes, and that's actually the most common and often the most comfortable setup. The settings are independent: data on the local eSIM, texts on the French line. Just check that outgoing texts are included in your plan from the country you're in, since outside the European Union they're usually charged per message.
Why does my text say "sent" when my contact hasn't received anything?
Because "sent" only means the network accepted the message, not that it was delivered. With dual SIM, this often happens when the message goes out on a tourist eSIM that has no messaging service. Our article on delivery receipts abroad explains this misunderstanding in detail.
How can I tell which SIM sent a message that's already gone?
Open the message, then its details: Android shows the carrier or SIM used, iOS indicates the line at the top of the thread. If in doubt, the itemised statement in your carrier's online account settles it for good, since only messages billed to that line appear there.
Should I disable data roaming on the French line?
Yes — outside the European Union it's the most cost-effective precaution you can take. Turning off data roaming blocks neither incoming texts nor incoming calls: you stay reachable on your usual number while removing any risk of out-of-plan data use.
Can a "data only" eSIM send texts?
No. These plans provide neither a usable phone number nor access to the carrier's message centre. To write, you need either your French line, an eSIM with voice and texting included, or an internet-based solution.
