Following issues with using data in Portugal a few years ago with excessive data usage on my iPhone, I did wonder if I should follow the same process I had used in October 2023 when I went to Amsterdam, last year when I went to Europe, and when I went to Helsinki in February.
The process I would use is go into Settings -> Mobile Data and turn off virtually all the apps which could use mobile data. I would turn off mobile data for virtually all my apps and also specifically iCloud Driveand iCloud Backup. I am pretty sure they were the culprits for my excessive data usage whilst in Portugal.
However this time I was heading to Calais and Dunkerque just for the day, though I was expecting to use my phone, I didn’t think I would go overboard on the data (even using iCloud for my photos) and I didn’t think I would exceed the 12Gb Fair Use limit.
When I arrived in France, my phone automatically connected to the correct network and I was able to use 5G (and I guess make phone calls, but I didn’t make any calls).
I did turn off data when I was on the ferry though (as recommended by DFDS) and connected to the Ferry WiFi.