How to avoid roaming bill shock: the phone settings to check before you fly

Why do roaming bills spike?

Most roaming surprises are not caused by one long video call. They come from data the phone uses on its own: photo backup after a day of sightseeing, app updates, cloud sync, messaging attachments, and video that plays automatically in social feeds. If that data runs over your home line on pay-as-you-go roaming rates, a few hundred megabytes can cost more than the rest of the trip's connectivity combined.

The second cause is the phone itself choosing a line. Dual-SIM phones can switch data to the other line when coverage is weak, and some features use mobile data when Wi-Fi is slow. Both are useful at home and expensive abroad if the other line is your roaming home SIM.

Step 1: Which line should carry your data?

Decide this before you touch any setting. There are three common set-ups:

• Travel eSIM for data, home SIM for calls and texts. The travel eSIM is the data line; the home line keeps your number but must not use data.
• Home line with a roaming pass. The home line carries data and you accept the pass price.
• Wi-Fi only. No mobile data at all; you rely on hotel, café and venue Wi-Fi.

Write the choice down. Every step below follows from it.

Step 2: How do you set this up on an iPhone?

Menu names change slightly between iOS versions and regions ("Mobile Data" in many countries, "Cellular" in the US), but the settings are in the same place.

1. Choose the data line. Go to Settings > Mobile Data (Cellular) > Mobile Data and select the line you chose in step 1.
2. Switch off mobile data switching. On the same screen, turn off Allow Mobile Data Switching so data stays on the line you chose. Apple notes that this setting also affects receiving calls on your other line while you are on a call; for cost control, off is the cautious choice.
3. Turn off data roaming on the home line. Tap your home line on the Mobile Data screen (on a single-line phone, open Mobile Data Options) and turn Data Roaming off. Apple's own guidance says to turn data roaming off to avoid roaming charges, and that an international data plan may need it on: so if a travel eSIM is your data line, its own Data Roaming setting usually needs to be on.
4. Reduce background use. In the data line's settings, set Data Mode to Low Data Mode. Scroll down the Mobile Data screen and turn off mobile data for apps that do not need it, such as photo backup or video apps.
5. Turn off Wi-Fi Assist at the bottom of the Mobile Data screen if you do not want the phone to use mobile data when Wi-Fi is weak.
6. Reset the usage counter. At the bottom of the Mobile Data screen, use Reset Statistics before departure so you can see exactly what the trip uses.

Step 3: How do you set this up on Android?

Android menus differ between manufacturers. These are the usual paths on Google Pixel and Samsung Galaxy phones; on other brands, search Settings for "roaming" and "mobile data".

1. Choose the data line. Pixel: Settings > Network & internet > SIMs, open the line you want and make sure Mobile data is on for it and off for the other. Samsung: Settings > Connections > SIM manager > Mobile data, and pick the line.
2. Turn off roaming on the home line. Pixel: in Settings > Network & internet > SIMs, open the home SIM and turn Roaming off. Samsung: Settings > Connections > Mobile networks > Data roaming.
3. Stop automatic switching. Some phones offer an option to switch mobile data automatically between SIMs; turn it off for the trip.
4. Turn on Data Saver. Pixel: Settings > Network & internet > Data Saver. Samsung: Settings > Connections > Data usage > Data saver.
5. Set a data warning or limit on the data line under its data usage settings, so you see a warning before a plan or a budget runs out.

Step 4: Which apps should be on Wi-Fi only?

The settings above stop the wrong line from being used. These app settings stop the right line from being drained.

• Photo and video backup: set to back up on Wi-Fi only.
• App and system updates: automatic downloads on Wi-Fi only.
• Messaging apps: turn off automatic media download over mobile data.
• Video and social apps: turn off autoplay on mobile data and lower the default video quality.
• Maps: download offline maps for your destination before you leave.

Step 5: What about calls, texts and voicemail?

Data roaming settings do not touch voice and text. If your home line stays on, ask your carrier three questions before you go: are incoming calls charged while roaming, are texts you send charged, and is checking voicemail charged? Some carriers bill missed calls that divert to voicemail. If the answers are expensive, our guide to Wi-Fi calling abroad and keeping your home number cheaply shows the alternatives.

Step 6: Have you set a spending ceiling?

Settings on the phone can be changed by accident; a limit at the carrier cannot. Ask for usage alerts and a roaming spending cap if your carrier offers them. A prepaid travel eSIM is also a ceiling in its own right: when its data or validity runs out, it stops, and it cannot add charges to your phone bill. Our guide to roaming caps and usage alerts explains the options.

What should you check after landing?

1. Look at the signal indicator for each line. The data line should show a network; the home line can show one too, which is fine if its data roaming is off.
2. Open Mobile Data settings and confirm the data line is still the one you chose. Updates and SIM changes can reset it.
3. Load a web page with Wi-Fi switched off to confirm data works on the right line.
4. Check usage on day two. If the home line shows any data used abroad, find out why before it grows.