Your phone can warn you at a threshold you choose, which is a useful backstop alongside the app's own low-data notification.
Android
- Open Settings, then Network and internet, then SIMs.
- Choose the SimFuse line.
- Open the data warning and limit settings.
- Set a warning, and a limit if you want the line switched off automatically.
iPhone and iPad
iOS does not have a per-line data limit. What it does have is a usage counter per line in Cellular settings, which you can reset when your plan starts so the figure means something.
A more useful habit
Check the app rather than the phone. It shows what the network has counted, which is what your plan is measured against. See "Check how much data you have left".
Turning off the biggest consumers
- Turn off automatic app updates over cellular.
- Turn off background app refresh for apps you do not need live.
- Download maps and playlists before you travel.
- Lower video quality in streaming apps, which is where most surprise usage comes from.