Here's the short version: when your dual-SIM phone won't use the data line you want—or keeps connecting through the wrong one—there are mainly three settings to check. (1) Which line is set as your mobile data line, (2) whether Allow data line switching is on or off, and (3) automatic switching during calls and similar situations. Reviewing these three settings is usually enough to get data running on the line you intended. If you're worried about surprise charges while traveling abroad, the safest first step is to turn (2) off so your phone won't quietly connect through another line (as of June 2026).
- When it won't switch, review three things: (1) the data line, (2) automatic switching, and (3) roaming
- If you're worried about accidental connections or surprise bills abroad, turn automatic switching off first
- Lock data to a single line and turn roaming off on your home line for peace of mind
- Bloomy's travel eSIMs are data-only—no phone number, voice calls, or SMS verification
- If it still won't work, toggle Airplane Mode, restart, and check eSIM activation and APN settings
This article lays out the exact settings on both iPhone and Android in a way that's easy to follow, even if it's your first time. If you'd like the basic setup steps themselves, take a look at our eSIM setup guide as well.
How is the "data line" decided on a dual-SIM phone?
Dual SIM means running two lines on a single phone (a physical SIM plus an eSIM, or two eSIMs, for example). Each line can receive its own calls and SMS, but only one line at a time is used for internet (data) traffic.
When people feel their phone "won't switch," it's usually because the data line is set to the line they didn't intend, or because the phone is automatically switching to another line under certain conditions. This matters even more when traveling: if data starts flowing over your home line, it's treated as international roaming and can get expensive—so it's worth checking these settings carefully.
If you haven't chosen an eSIM for your destination yet, it helps to first browse our eSIM comparison page to get a sense of country, data amount, and number of days—so the setup makes more sense afterward.
The three settings to check first
(1) The line used for mobile data (your primary data line)
This is where you choose which line handles your data. If you're using a travel eSIM, set this to that eSIM. When this is pointed at the wrong line, you'll be left thinking "I set it up, but it still won't switch."
(2) Allow data line switching
This lets the phone automatically hand data off to the other line in situations such as during a call. If you want to avoid connecting through an unintended line (and the bills that can follow), it's safest to turn this off.
(3) Data roaming on/off
Check the roaming setting for each line. A travel eSIM uses local networks, so roaming may need to be on; meanwhile, turning roaming off on your home line while abroad makes it easier to avoid accidental connections. For more on how roaming works, related articles in our eSIM guide index are a helpful reference.
Where to find these settings on iPhone
On iPhone, go to Settings → Cellular (called Mobile Data in some regions). The exact wording may differ slightly depending on your iOS version.
- Open Settings → Cellular
- Tap Cellular Data and select the line you want to use for data (your travel eSIM)
- On the same screen, turn off Allow Cellular Data Switching (to prevent accidental connections)
- Tap each line and set Data Roaming as needed
- For the line you don't need, switching Turn On This Line off keeps things even more cleanly separated
Labels like "Secondary" and "Primary" sometimes refer to the default line for calls and iMessage, which is managed separately from your data line. Always confirm data under the Cellular Data item.
Where to find these settings on Android
On Android, the names and menu paths vary by manufacturer, but you'll generally find them under Settings → Network & internet → SIMs or Mobile network.
- Open Settings → Network & internet → SIMs (or SIM cards / Mobile network)
- Under Mobile data, select the line you want to use (your travel eSIM)
- If there's a setting to switch the data line automatically, turn it off
- In each SIM's details, set Roaming as needed
- Temporarily disabling the line you're not using makes accidental connections less likely
If you're not sure whether your device supports eSIM and dual SIM, it's worth checking first with our device compatibility checker.
Quick reference (when it won't switch or you're worried about accidental connections)
| Setting to check | Where on iPhone | Where on Android | Recommended state |
|---|---|---|---|
| Line used for data | Cellular → Cellular Data | SIMs → Mobile data | Select your travel eSIM |
| Automatic line switching | Allow Cellular Data Switching | Automatic data switching | Off (prevents accidental connections) |
| Data roaming | Each line → Data Roaming | Each SIM → Roaming | On for the travel eSIM as needed; off for your home line |
*Setting names and on-screen layouts can change with your device or OS updates. For the latest, check your device manufacturer's guidance (as of June 2026).
If it still won't switch or won't connect
- Turn Airplane Mode on for about 10 seconds, then off to re-establish the connection
- Restart your device (when setting changes don't seem to take effect)
- Confirm that your travel eSIM has been activated at the right time. Depending on the plan, some eSIMs are meant to be activated after you arrive at your destination
- Check whether your plan requires an APN (access point) setting
- Switching can struggle in areas with no signal or weak coverage, so try moving to a different spot
If you've worked through all the steps and still can't connect, our connection troubleshooting page gathers things to check by situation. If that doesn't resolve it, you can reach us through our contact page.
Tips for avoiding surprise bills
The key to avoiding connections over an unintended line is the combination of locking data to a single line, turning automatic switching off, and turning roaming off on the home line you're not using. Keeping your phone in this state while you're abroad helps ease the worry of unknowingly running data on another line and watching the bill climb.
Note that Bloomy's travel eSIMs are generally data-only. If you need a phone number, SMS (including SMS verification), or voice calls, it helps to keep a physical SIM (with your existing number) for calls and texts, or to arrange a separate option that includes a number. With dual SIM, it's easy to split things as "data on the travel eSIM, phone number on your original SIM."
What you can do with Bloomy
With Bloomy, you can choose an eSIM to match your destination, data amount, and number of days. When you want to confirm after setup that you're really connecting through the line you intended, you can check your data usage and remaining balance from My Account. If you're unsure which plan to pick, use the eSIM comparison page to compare by country, data amount, and days. Data guidance and conditions of use can vary by country and region, and pricing is shown in USD—so please confirm the latest details at the time of purchase.
Please also keep in mind that real-world speeds and connection quality depend on the local network, your device, and the area you're in, and that all plans are subject to a fair-usage policy.
Summary: what to do next
When your dual-SIM phone won't switch data lines—or you're worried about accidental connections—the basics are to review three things: (1) the line used for data, (2) automatic switching (off recommended), and (3) roaming. For the basic setup steps, see our setup guide; if it isn't resolved, head to troubleshooting. And if you're looking for an eSIM for your destination, start from the comparison page.
