Iphone settings to allow phone/text but only WiFi for everything else?
 

I know this is probably a question with an intuitively obvious answer, but the more I google the more convoluted instruction sets I find, leading my down the "parental control" rabbit hole, etc.

Here's the situation. I'll be traveling in an area where my roaming will incur charges, and since I'm on vacation and will have WiFi most of the time I don't really need to bother with a local SIM. I can catch up on email at night, but I want to have phone/text access at all times. I get few of either, but don't want to be totally without for stretches of time. I'm also set up to use WiFi for phone/text when available, so I'm really only talking about a few hours a day where I'd like to use data for phone/text.

So what I want is:

phone to behave normally when on the Wifi (most of the time)
restrict data to Phone/Text only all of the time.

is it just as simple as turning the iphone "Cellular Data" to OFF, or will some things beyond phone/text still come through. (I don't want accidentally pay for things like app updates!)

Voice Roaming -> On
Data Roaming -> Off

I'd also suggest enabling WiFi calling before you leave and enabling "Prefer WiFi While Roaming" which will help minimize roaming costs if you get/make a call while connected to WiFi.

I don't have a Voice Roaming option, only Data Roaming. I'm assuming that turning off Data Roaming, will restrict it to Voice only.

This a T-Mobile phone, and the menus I see are:

Settings>>Cellular>>Cellular Data (on/off)
or
Settings>>Cellular>>Cellular Data Options>>Data Roaming (on/off)

I'm thinking that setting either of those options to OFF might accomplish the same thing.

I already have the use WiFi when available option turned on.

One reason I feel like such a noob is that I have T-Mobile Simple Choice and this is a very rare trip to a country not included in my plan. Haven't worried about data usage in years.

For me, it's Settings -> Cellular -> Cellular Data -> On and
Settings -> Cellular -> Cellular Data Options -> Roaming -> Voice & Data Roaming -> On to expose those two sub-options.