Here's the short version: if your dual SIM isn't using the data line you expect, or it keeps connecting on the wrong line, there are mainly three settings to check. (1) Which line is set as your data line (primary), (2) whether "Allow data line switching" is on or off, and (3) automatic switching during calls or other specific conditions. Reviewing these three usually gets your data running on the line you intended. If you're worried about unexpected charges while traveling abroad, the safest first step is to turn (2) off so your phone won't quietly switch to another line (as of June 2026).
This guide breaks down where to find these settings on both iPhone and Android, in plain language for first-time users. If you also want the basics of setting up an eSIM in the first place, see our eSIM setup guide.
How does a dual SIM decide which line carries your data?
Dual SIM means running two lines on one phone (a physical SIM plus an eSIM, or two eSIMs). Each line can receive calls and SMS, but only one line at a time is normally used for internet (data) traffic.
When data "won't switch," it's usually because the data line is set to the line you didn't intend, or 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 abroad, it's treated as international roaming and can get expensive, so it's worth double-checking these settings.
If you haven't picked an eSIM for your destination yet, it helps to browse our eSIM comparison page first to get a feel for countries, data amounts, and durations before you dive into settings.
The three settings to check first
1. The line used for cellular data (primary)
This sets which line handles your data. If you're using a travel eSIM, choose that eSIM here. When this points to the wrong line, you'll feel like "I set it up, but it still won't switch."
2. Allow cellular data switching
This lets your phone automatically switch the data line to the other one in certain situations, such as during calls. If you want to avoid using an unintended line (and the charges that can follow), it's safest to turn this off.
3. Data roaming on/off
Check whether roaming is allowed on each line. A travel eSIM uses a local network, so roaming may need to be on for it; meanwhile, turning roaming off on your home line while abroad helps prevent accidental connections. For more on how roaming works, see the related articles in our eSIM guide.
Where to find these on iPhone
On iPhone, open "Settings" → "Cellular" (sometimes shown as "Mobile Data"). The exact wording can vary slightly by iOS version.
- Open "Settings" → "Cellular"
- Tap "Cellular Data" and choose the line you want 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, turning off "Turn On This Line" keeps them more reliably separated
Labels like "primary" and "secondary" can 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 on Android
On Android, the names and menu paths vary by manufacturer, but generally you'll go to "Settings" → "Network & internet" → "SIMs" or "Mobile network."
- Open "Settings" → "Network & internet" → "SIMs" (or "SIM cards" / "Mobile network")
- Under "Mobile data," choose the line you want to use (your travel eSIM)
- If there's an automatic data-switching option, turn it off
- In each SIM's details, set "Roaming" as needed
- Temporarily disabling the line you aren't using helps avoid accidental connections
If you're not sure whether your device supports eSIM and dual SIM, it's worth confirming first with our device compatibility check.
Quick reference (for switching issues or accidental-connection worries)
| Setting to check | On iPhone | 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 | Auto switch / automatic data switching | Off (prevents accidental connections) |
| Data roaming | Each line → Data Roaming | Each SIM → Roaming | On for travel eSIM if required; off for home line |
* Setting names and menu layouts can change with device models and OS updates. For the latest details, check your device maker's guidance (as of June 2026).
Still not switching or not connecting?
- Toggle 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)
- Check that your travel eSIM is 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 manual APN (access point) settings
- Switching can fail in areas with no signal or weak reception, so try moving to a different spot
If you've worked through these steps and still can't connect, our connection troubleshooting guide covers what to check for different situations. If that still doesn't help, you can reach us through our contact page.
Tips to avoid surprise roaming charges
The key to avoiding traffic on an unintended line is the combination of "lock data to one line" + "turn automatic switching off" + "turn off roaming on the home line you're not using." Keeping your phone in this state while traveling reduces the worry that another line quietly racks up charges without you noticing.
Note that Bloomy's travel eSIMs are data-only: they don't include a phone number, SMS (including SMS verification), or voice calls. If you need a phone number, SMS, or calls, it helps to keep a physical SIM (with your existing number) for calls and texts, or arrange a separate option that includes a number. With dual SIM, a common setup is "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. After setup, if you want to confirm you're connecting on 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 usage terms can vary by country and region, so please confirm the latest details shown at the time of purchase. As with any mobile service, connection quality depends on the local network, your device, and your location, and a fair-use policy applies.
Summary: what to do next
When your dual SIM won't switch data, or you're worried about accidental connections, the basics are to review three things: (1) the line used for data, (2) automatic switching (off is recommended), and (3) roaming. For the basic setup steps, see the setup guide; if things don't resolve, head to troubleshooting. Looking for an eSIM for your destination? Start from the comparison page.
