Why am I able to receive SMS texts when Cellular off and no WiFi connection?
Why am I able to receive occasional SMS texts and inbound call attempts, despite cellular being off and not being connected to a WiFi network?


Cellular data is not related to SMS.  That's a function of your voice service. See:  Cellular iPad-Air text message configuration for more details.

WiFi calling is a supplement to your cellular voice plan.  If the WiFi network is available, your phone will use WiFi instead of the LTE network to make a call.  If you turn off WiFi and cellular data and leave cellular voice on, calls will continue to come through.

But to summarize it, if you can make voice calls on your cell network, you can receive SMS/MMS messages.