Earlier quoted context omitted.
At least if you’re travelling to Europe there are far cheaper offers with local carriers for about €/£ 5 for 20GB/30days valid across the EU and often UK. Lycamobile and Simyo usually have good deals though ended up needing to get a physical SIM in Spain as they require showing ID.
I forget if it was with Lyca or a similar brand, but they would deduct 5 cents for each sms before you setup a plan. I was incredibly careful to not do anything and disabled almost everything but apparently iPhone phones home in a way that’s totally hidden from the user and most providers hide and zero the charge but not these rando brands. Another 5 EUR down to tube because I only had 4,95 for my 5 EUR plan. Dunno w…
It's not "totally hidden". You need to be extra careful to cancel the request to add the new SIM number to FaceTime/iMessage when prompted.
The prompt appears every time the phone is restarted though, and whenever the SIM is switched to.