Allow Cellular Data Switching: what it does, and why it matters on a trip

What the iPhone option really does

The switch is in Settings, Cellular (Mobile Data in some regions), Cellular Data, at the bottom of the list where you pick the data line. Apple's Dual SIM article describes two jobs for it.

The general one: turn it on if you want your iPhone to use cellular data from both plans, depending on coverage and availability. The specific one is about calls. With two lines, only one carries data. If you are on a call on the other, voice-only number, the switch lets that number carry voice and data during the call. Turn it off and, Apple says, cellular data will not work while you are on that call.

Apple adds a footnote that matters abroad: that switch during calls will not work if you are currently using data roaming. It does not say the same about the general use, so the safe reading is that the option can still move data to your other line in some cases.

The Android versions, maker by maker

• Google Pixel: Automatic data switching, in Settings, Network and internet, SIMs. Google requires it, together with mobile data on both SIMs, for simultaneous calls on the Pixel 8a and later.
• Samsung Galaxy: in Settings, Connections, SIM manager. Samsung describes it as using another SIM card for data services if the preferred SIM card cannot connect to the network.
• Sony Xperia: Automatic data switching switches to the other SIM when one is out of its service area, and a further option can switch when the transmission speed becomes slow.

Google also notes, about the default SIM, that most data goes through the default SIM for that use type. Most, not all. Names and options vary by model and Android version, so if yours does not match, type data switching into the search box at the top of Settings.

Why it can spend data from your home line

Put the descriptions together and the risk is clear. On a trip, the travel eSIM is your data line and your home line is the backup. The moment the travel line loses coverage, cannot connect, or, on some phones, gets slow, a phone with switching on is allowed to move data to the home line.

Whether that costs anything depends on your operator. Inside the EU your home plan may roam at domestic prices; outside it, the data is billed under your operator's roaming terms. The same thing can happen when the travel eSIM runs out of data, as explained in what happens when your travel eSIM data runs out.

The phone does not always tell you. On iPhone the two rows of bars show which line carries data at the top, but few people check that in the middle of a day out.

The setting that makes switching harmless

There are two ways to close the gap, and the second is the one that covers every case. The first is to turn the switching off for the trip. The cost is small: on iPhone, mobile data pauses during a regular call on your home number, while calls over WhatsApp or FaceTime use the data line and are not affected.

The second is to turn data roaming off on your home line. With that off, the line cannot use mobile data abroad at all, so it does not matter whether switching, a call or anything else tries to move data onto it. Calls and texts on your number keep arriving, because they do not need data roaming. The steps for each phone are in how to turn off roaming on your home line.

Doing both is the tidiest set-up. The travel eSIM, Snoozim's included, stays as the only line that can carry data, with data roaming on.

How to tell if it has already happened

Look at the data counters for each line. On Android, each SIM has its own usage figure under the SIM settings; on a Galaxy, under Connections, Data usage. On iPhone, Apple notes that on Dual SIM the usage shown is for the selected cellular data number, so switch the selection briefly or check your operator's app for the home line.

If the home line shows new usage since you landed, some data went that way. Your operator's app or account will show what it cost. Our guide to checking data usage explains how to reset the counters on arrival, which makes this check much easier.