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.
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.
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?