Phones use mobile data in the background for app updates, photo backups, and sync, even while you are not using them. On a travel eSIM this can quietly use part of your allowance. Turning on Low Data Mode (iPhone) or Data Saver (Android) for your Vuoly line stops most background use. Your connection is not slower with these settings on: browsing, maps, and messaging work normally, and only background transfers are paused.
On iPhone
Go to Settings > Cellular (called Mobile Data in some regions).
Tap your Vuoly plan.
Turn on Low Data Mode.
On Android
Go to Settings > Network & internet > Data Saver (on Samsung: Settings > Connections > Data usage > Data saver).
Turn it on.
Extra savings
Turn off automatic app updates over mobile data in the App Store or Google Play settings.
Streaming and video calls use the most data. When good WiFi is available, use it for those.
To see what you have used so far, check: How to check your data usage.