Mobile Data Roaming off, still get charged

I'm starting this thread again as it appear this seems to happen to a number of people and I wanted to share my experience of this.

I have two phones, one with iOS 16.7 and the other with iOS 17. One has an eSIM (iOS 17) and the other doesn't. Both phones are registered in the UK. My UK provider charges a flat fee for roaming which is automatically triggered if roaming is used, or a call/SMS is initiated from roaming country to UK. Roaming is turned off on both phones.

On a recent trip to the US, as soon as I landed, I got the 'welcome' SMS message then immediately got charged because my phone sent a very small amount of data (1KB). Looking back over the billing, no further data was sent that day. What I did see just after I landed was: The screen went translucent black and a full screen message 'Your Primary SIM wants to send an SMS message'. This flicked up for less than a second. There was a 'cancel' and 'accept' option, but this was ignored. Immediately after this message, I got the SMS that I would be charged the daily rate for that day.

So it seems, this issue is not related to data roaming but to the Primary SIM to want to send an SMS message (presumable back home), which triggers the charge from the provider, but appears on the providers bill as a very small (1KB) amount of data.

The only way I've found to stop this is to disable the SIM itself as data roaming has nothing to do with sending of SMS and the phone appears to want to do this automatically. I've spoken to my provider and they are aware of the issue and have confirmed it only appears to affect Apple users so iOS is doing something that Android doesn't.