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 Drive and iCloud Backup. I am pretty sure they were the culprits for my excessive data usage whilst in Portugal. This time (as an experiment) and as I was away only for a short time, I only turned off mobile data for iCloud Drive and iCloud Backup.
This trip I was taking the Eurostar from London St Pancras to Brussels. Obviously the train would have to travel through France before before arriving in Brussels in Belgium.
After travelling through the tunnel, 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). As I crossed the border into Belgium I was switched automatically to a different network in Belgium.
I had no problems with data, texts and calls in Belgium and on my return visit there was another short stint with a French provider before entering the tunnel and rejoining Three when I was back in the UK.