I have a SIM card that I use only to receive SMS.
It does not do not include mobile data in the plan, and it is "pay as you go". This means that as soon as my phone connect to the internet, even to transmit/receive 1 bit, I get charged 1€ for the day.
I used this SIM card with both a Nexus 6P and a Pixel 1, with:
- Mobile data disabled
- MMS disabled (no auto download) and never sent/received
- Roaming disabled
and still both phones use mobile data and I get charged 1€ every day!!
I tried with my ancient Nexus 4 and everything work as expected: mobile data disabled, no mobile data used, and no charges.
What are my newer phones sending/receiving?! Is there a way to stop this?