How to disable phone calls and text messages in iOS / on iPhone? I don‘t use mobile phone call and text messaging any longer but I need to keep cellular data / internet running since FaceTime, WhatsApp replace sms and cellular calls with better quality.

To implement that strategy I currently use a secondary sim card from my father. The upside is that it’s entirely free, the downside is that I get his phone calls, too. I plan to move to an individual solution some time in the future, this setting was to test whether or not pure-internet communication mode would be feasible. 

Is there any way you are aware of to entirely and cleanly switch off the phone and text-message functions of the iPhone without affecting any other functions?
Some carrier can provide sim card that cellular data only.
Mainly used  for iPad notebook and IoT device .
You can use it for your iPhone to get the results you want.