When I'm in range of WiFi (95% of the time), turning OFF both Apple Intelligence and Cellular Data eliminated the excessive cellular data use for me. I'm on a low-cost prepaid cellular plan with a Verizon MVNO, so I monitor cellular data usage.
There are a few similar threads posted in Apple Support Communities in the last few months, including these two where I've also posted responses:
Excessive General data use over cellular network while in nightstand mode
Unread, Why is System Services consuming excessive data on iPhone 15 Pro Max?
Here's why I conclude that Apple Intelligence is using huge amounts of cellular data:
1) I did not have excessive cellular data use until I upgraded to an Apple refurbished iPhone 15 Pro in mid May 2025. My prior iPhone 12 Pro did not have Apple Intelligence capability. After the upgrade, I used about 4Gb of cellular data in less than 2 months, instead of my typical usage of 150-250Mb per month.
2) Apple Intelligence is NOT listed separately among the detailed Settings>Cellular>Cellular Data>System Services>Cellular data usage list. General shows over 4Gb used. No other Cellular Data service listed used more than 100Mb over that data collection period.
3) Turning off both Apple Intelligence and Cellular Data fixed the problem for me.
When I am out of WiFi range, traveling and using Apple Maps, Mail, Contacts, and a few other low-data apps, I DO turn on Cellular Data, but I leave Apple Intelligence 'off'. I have not experienced excessive data consumption since.
As long as WiFi is available, turning off Cellular Data has no effect on my ability to use my iPhone as a smartphone.
I'm going to submit a 'bug report' to Apple, recommending that Apple Intelligence cellular data be listed separately instead of included in General, and add a toggle that forces Apple Intelligence to use Wifi, instead of allowing unrestricted cellular data.
