How to choose which line uses mobile data on your Apple iPhone 17 Pro
An iPhone with two lines still picks only one of them to carry data. Everything else — which number rings, which one texts — is set separately, so a phone can be using a brand-new travel eSIM for calls while quietly spending your home carrier's data in the background. This is the setting that decides it.
- Step 1 of 5
Open Settings.
SettingsPhoneMessagesSafariCameraPhotosMapsClock - Step 2 of 5
Tap Cellular.
Mobile Service in British and Australian English.
Settings
GeneralAccessibilityCameraCellularSearchScreen Time - Step 3 of 5
Tap Cellular Data.
The line named beside it is the one carrying your data right now. If that is not the line you meant, this is where it changed without you noticing.
SettingsCellular
CellularFind out how much data you're using, set data restrictions, and manage carrier settings such as eSIM and Wi-Fi calling.
Cellular DataPrimaryPersonal HotspotOffCellular Data OptionsTurn off cellular data to restrict all data to Wi-Fi, including email, web browsing, and push notifications.
Default Voice LinePrimary - Step 4 of 5
Tap the line you want to use for data.
Allow Cellular Data Switching lets the iPhone fall back to the other line where coverage is better. Apple notes it does not switch while data roaming is in use — so abroad, treat the line you pick here as the one you are on.
CellularCellular Data
PrimarySecondaryAllow Cellular Data SwitchingYour iPhone can use either line depending on coverage. Switching does not happen while data roaming is in use.
- Step 5 of 5
Swipe up from the bottom edge of the screen to return to the Home Screen.
SettingsPhoneMessagesSafariCameraPhotosMapsClock
Your Apple iPhone 17 Pro will list more settings than these illustrations show — the row you want is the one ringed. Wording shifts with your language region and sometimes with your carrier, so the notes call out the common alternatives.