In the same menu where you installed the eSIM — set mobile data to the travel line, and leave calls and messages on your home line
A dual-SIM phone runs two lines at once but sends data down only one of them. Which one is a setting, and it is not chosen for you correctly by default.
iPhone
- Settings › Mobile Data
- Tap Mobile Data at the top
- Choose the travel eSIM
- Turn off Allow Mobile Data Switching
Android
- Settings › Network & internet › SIMs
- Find Mobile data and choose the travel line
- Leave Calls and SMS on the home line
- Samsung: the same choices live in SIM manager
Why the Switching Setting Matters
Mobile data switching lets the phone borrow the other line when the chosen one has weak signal. At home that is helpful. Abroad, the other line is your home SIM at roaming rates, and it will do this silently in any spot where the travel network is poor.
Turn it off before you fly. It is the reason people who did everything else correctly still come home to a roaming charge.
Naming the Lines
Give the travel line the name of the destination rather than leaving it as "Secondary" or "Business". Every screen that asks which line to use becomes obvious, and so does the operator name at the top of the display.
If you are unsure which line is carrying data right now, the mobile data screen shows usage per line. The one whose counter is moving is the one in use
