How to turn everything off EXCEPT wifi

Hi,

I'm frequently travelling with my iPhone and never figured how to do the following :

Turn off everything except WiFi

I can turn off the data when outside of my zone, but I still get connected to a mobile phone provider wherever I go.

One could argue that I could simply NOT answer my phone when getting incoming calls, which is true, indeed, but I can not refuse incoming SMS, and I get charged for that.

SO, in the end, please help : how can I very simply turn everything off BUT the WIFI.

Thanks 🙂