How to block all calls and texts except for ones in contact list

Restrictions does not have a feature to block all calls and texts except ones in contact list. I am looking to filter and only allow incoming and outgoing calls and texts for the few contacts in my son's iPhone. He needs a phone but have things limited. He is autistic and I need to be able to reach him but in the same since he is easily bullied and other use his phone and make prank calls. (Anyway this is being handled just wanted to mention why so no need for bashing)

Does anyone know of a 3rd party app that may work? Our carrier doesn't have this feature either. I also looked at Qustidio and it does not support the function on iOS devices. Any assistance or direction would be great.