Put iPhone Sim card in iPad Cellular. I thought iPad will replace my iPhone?

I am totally confused by iPad cellular.

If I put iPhone sim card in iPad cellular, how to make call, how to receive call.

I know iPad wifi+cellular for long time, never thought how it work, only what I thought I should work, until I bought iPad cellular, which is totally useless to me.

If I care how to use iPad WIFI to browse, I can set up iPhone hotspot to connect iPad to internet.

Why should I buy iPad cellular? iPad cellular can only do data, if I put my sim card into iPad…. then, how to make call???

1) iPad can’t dial/receive call.

2) Even though, somehow set it up (iPad can receive call and dial, it seems complicated). iPhone, what’s the use of my iPhone without sim card?