To expand a little bit on what chrfr said (which I agree with)... the way in which the sharing is accomplished is by the iPhone sending packets of sound data back and forth to/from the iPad through a network data connection (typically local wifi), which is what allows the iPad to appear be acting as a phone. This is only possible because the iOS software has been engineered to make that possible between an iPhone and an iPad (the iPhone knows that the iPad is accessible, it knows that the proper permissions are in place, etc). It is one component in a closed system talking to another component in that same system.
Asking for a landline to share to an iPad is a whole 'nother level of complexity: typical landlines don't have a wifi connection, so the routing to the iPad would have to be done at some trunk level up the tree from the landline provider, the iPad would have to be able access the voice cellular network to take the call (which its hardware may be able to do, but which typically isn't enabled in iOS for an iPad), Apple wouldn't have end-to-end control of the connection between landline and iPad (like they do between an iPhone and an iPad), too many hardware configurations to support (which Apple hates), and on and on.
I don't fault you for wanting it, but very unlikely to ever happen, IMHO.