Why is System Services consuming excessive data on iPhone 15 Pro Max?

I have an iPhone 15 Pro Max using iOS 18.5.

I have a plan with limited data and normally this isn’t an issue - I am rarely out of Wifi coverage. The last 4 times I have been out of Wifi coverage, my phone has eaten all of my data - each time I have topped it up again and tried various fixes.

I am using Low data mode, data is off for all apps, I have turned off location services for everything except “Find my”, “Emergency calls and SOS” and “Setting time zone”; the only apps having access to location services are switched to “While using” and were not being used at the time of data drain.

I took a screen shot of my data usage at 4:35pm (21.4Mb) and another at 10:29pm (1.03Gb) and my phone had been turned off for about 2 of those hours. When I look at the data usage, it is mainly System services/General (1.02Gb)

You can see from the screenshots of messages from my cell phone provider that the data is disappearing rapidly - 204Mb in 1 minute on 6 May and 414Mb in 7 minutes on 20 May. The 414Mb disappeared while I was playing music that had already been downloaded onto my phone (it has been there for several months). I know that the usage on 20 May was after I had restarted my phone (it was turned off for about 2 hours while I was out).

Other than turning cellular data off altogether, how do I fix this???

Image 1 - before data drain

Image 2 - after data drain

Image 3 - "System services/General" is the culprit!

Image 4 - texts from provider showing the rapidity of the data drain (414Mb in 7 minutes)

[Edited by Moderator]