Background
I'm using an iPhone Xs Max, iOS 13.3.1.
I typically keep celluar turned off, and WiFi on. When I'm at home I automatically connect to my WiFi and can receive calls and texts no problem. When I'm not home, I can't receive calls or texts, which is fine by me — and I'll only turn cellular on if I have to use Uber/Lyft or if I need to make an outbound call.
Issue
On very rare occasions when I'm away from home (and therefore connected to neither cell nor WiFi) I'll receive an SMS text. Usually just random stuff like Amazon sending me a locker access code, telling me I have a package to pickup, etc.
On even rarer occasions I'll receive a call in this mode. I can't actually answer it, but it'll ring once briefly and show me a caller ID.
Question
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?
Best Answer
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.