Mobile data vs data roaming: two switches, two jobs

Two switches answering different questions

Apple describes the first one plainly: you can turn cellular data on or off to limit apps and services from using the cellular network, and when it is off, apps use only Wi-Fi. Google's list of Pixel settings says the same in fewer words: Mobile data turns mobile data on or off. It does not care where you are. Off at home or off in Rome, the result is the same: no internet without Wi-Fi.

Data roaming is narrower. Apple says it permits internet access over a cellular data network in regions not covered by your carrier's network, and Google describes it as letting your phone send data over other carriers' networks when you leave your carrier's area. It only comes into play when the line is on someone else's network.

Apple's own roaming steps show the order: turn on Cellular Data first, then tap the line, then Cellular Data Options, then Data Roaming. The first switch opens the door; the second decides, line by line, who may go through it abroad.

Where each switch lives

• iPhone with one line: Settings, Cellular (Mobile Data in some regions), the Cellular Data switch at the top. Data Roaming is under Cellular Data Options.
• iPhone with two lines: Settings, Cellular, Cellular Data to turn it on and pick the line. Data Roaming is inside each line, under Cellular Data Options.
• Pixel: Settings, Network and internet, SIMs. Mobile data is the switch on that screen; tap each SIM to find its own Roaming switch.
• Other Android phones: type roaming or mobile data into the search box at the top of Settings.

The names follow the language of the phone. On an iPhone in Spanish they are Datos móviles and Itinerancia de datos; on a Pixel in Spanish the second one is simply called Roaming. If you read instructions in one language and your phone is set to another, that is often the whole confusion.

With two lines there are three settings, not two

Once a travel eSIM sits next to your usual line, a third setting joins in: which line carries data. Apple says one number at a time can use cellular data, chosen in Settings, Cellular, Cellular Data. Google says only one SIM can be the default for data, chosen under Your primary SIMs in the SIMs screen.

So the phone checks three things before a page loads. Is mobile data on? Which line is the data line? Does that line have data roaming on, if it is roaming? Each one can stop the connection on its own, and each one is a separate place in Settings.

Roaming being per line is what makes a trip setup work. Apple's Dual SIM steps say to tap the number you want to modify and change its options, so your usual line can keep roaming off while the travel eSIM has it on. The reasons for each half are in do you need data roaming on for a travel eSIM and how to turn off roaming on your home line; this article is about how the switches combine.

Every combination abroad, with a travel eSIM and your usual line

• Mobile data off: no mobile internet on either line, whatever the roaming switches say. Calls and texts on your number are not affected.
• Mobile data on, eSIM is the data line, eSIM roaming on, home roaming off: the setup you want. Data goes through the eSIM and the home line cannot use data abroad.
• Mobile data on, eSIM is the data line, eSIM roaming off: bars on the eSIM, but nothing loads.
• Mobile data on, home line is the data line, home roaming off: nothing loads and the eSIM counter stays at zero.
• Mobile data on, home line is the data line, home roaming on: you are browsing on your operator's roaming terms.
• Mobile data on, eSIM is the data line, both roaming on: it works, but data can slip onto the home line through data switching.

The last case is the sneaky one. Apple's Allow Cellular Data Switching and Google's Automatic data switching let the phone use the other line in some situations, explained in allow cellular data switching.

Switching off mobile data does not fix roaming

A common reflex before a trip is to turn Mobile data off so the phone cannot roam. It works, in the sense that nothing roams, but it also switches off the travel eSIM, because the master switch covers every line. Apple's roaming article for Spain even notes that with both mobile data and data roaming off, the mobile data icon should not appear in the status bar at all.

The precise tool is the roaming switch on the home line. With that off, your usual line cannot use data abroad, while the eSIM line, with its own roaming on, carries everything. On a prepaid travel eSIM, Snoozim's included, having roaming on adds no charge, because the data is paid in advance.

If what you want is to spend less data rather than to stop roaming, use the per-app switches instead. Apple says that when cellular data is off for an app, that app uses only Wi-Fi. Our guide on stopping automatic updates on mobile data covers the usual suspects.

At home, and on the way back

With only your usual line in your own country, the roaming switch normally does nothing, because you are on your operator's network. Apple adds one caveat: depending on your carrier, data roaming might be used when you travel domestically, so if your operator relies on partner networks in some areas, ask before switching it off for good.

When you get back, three changes undo the trip setup. Set the home line as the data line again, turn the travel eSIM line off, and leave mobile data on. The home line's roaming switch can stay off until your next trip; inside your own network it is not needed. If you travel within the EU, check what your own plan includes there in roam like at home explained.

To confirm that the travel line really carries the data on the next trip, the checks are in how to know if your eSIM is working.