How do I use one iphone as extension of main iphone via wifi

I have a main iPhone with SIM and a second old iPhone with non SIM. The second iPhone serves as a copy of my calendar and contacts at another location in the house. I notice that when my main iPhone rings, my second often - but not always - rings and I can answer the call on that second phone. This was accidental for me. How do I set it up so that secondary iPhones with no SIM can be a dependable extension of my main iPhone? It's very handy like having an extension on a landline.