Hello World! I originally discovered this defect when I bought my iPhone with iOS 12.2, and have noticed the defect continues with iOS 12.3 & 12.3.1. This defect is discoverable when using a prepaid mobile data SIM with Cellular Data switched OFF in System Settings and using WiFi all the time. My cellular carrier sends me a SMS/text message whenever I supposedly attempt to use cellular data services.
Based on my research, my iPhone attempts transmission of a data packet intermittently & randomly when on WiFi and Cellular Data is switched OFF in System Settings. Also, when doing a cold power up, my iPhone will attempt to transmit a data packet via cellular data!
My quick fix, is to activate "Airplane Mode" and then, manually activate WiFi & Bluetooth.
Apparently, there is no way to inform iOS that my cellular data SIM is prepaid vs postpaid, or cellular data is metered.
Those using postpaid cellular data, or unlimited cellular data, are NOT affected by this iOS defect.
I decided to make this post because iOS 12.4 is currently being beta tested while iOS 13 is just around the corner.
Have a Great Day, Everybody!