Making and receiving calls on an apple device (an iPhone with no cellular account) using another iPhone's cellular account.

On my iPad I am able to make and receive calls using my iPhone's cellular account when my both devices are on the same wi-fi network.

Some of us has a few iPhones laying around without a cellular contract. I don't understand why apple does not let iPhones use the same feature as iPad's. I would like to be able to make and receive calls on an iPhone with no cellular network using my active iPhone's cellular account. Just like my iPad pro.

Is that difficult?