One line is normally selected for mobile data. The second line can still handle calls and texts, but it does not usually provide a separate simultaneous data connection.
The direct answer
Your phone uses the SIM or eSIM selected as the mobile-data line. Both numbers may remain active for calls and messages, but normal app traffic is routed through one mobile network at a time.
How to identify the selected line
iPhone
Open the current mobile-service settings, then select Mobile Data. Apple changes menu wording between releases, so follow the current labels on the device.
Pixel
Open Settings > Network and Internet > SIMs, then choose the primary SIM for mobile data.
Samsung Galaxy
Open Settings > Connections > SIM manager, then check the preferred SIM for mobile data. Wording can vary by model and One UI version.
Verify instead of assuming
- Turn off Wi-Fi.
- Note the selected data line in settings.
- Open a simple webpage.
- Check the data indicator and the usage screen for that line.
- Switch the data line and repeat if you need to compare coverage.
What automatic data switching changes
An iPhone can offer Mobile Data Switching, while supported Android phones may offer data during calls or automatic switching. These features do not combine both plans into one faster connection. They allow the phone to use the other line in a defined situation.
Automatic switching can create unexpected costs, particularly abroad. Check roaming and data allowances on both lines before enabling it.
Data during a call
On some phones, a call on the non-data line can interrupt data or cause the phone to use that line temporarily. Google documents a Data during calls option on supported Pixel devices. Test this with an ordinary call if simultaneous calling and online access are important to your work.
If data comes from the wrong line
- Confirm the intended SIM is enabled.
- Check the mobile-data default.
- Turn off automatic switching temporarily.
- Review Data Roaming on both plans.
- Restart the phone after a recent SIM or eSIM change.
- Contact the relevant network if the selected line has signal but no data.
Prove the active data route with Wi‑Fi off
Open the SIM manager, note the selected data line, disable Wi‑Fi and load a small page. Check the data counter for that line, then place an ordinary call and observe whether the phone changes data routes.
- Keep automatic data switching off until its cost is understood.
- Repeat the test after a restart.
- Check roaming separately for each line.
Sources checked
These sources were checked on 25 July 2026. Network features and menu names can change, so confirm the exact device, software version, network and plan before relying on them.
