Just want to throw out another option. They make Bluetooth phones. I got my parents a set years ago so that they would hear the phone ringing no matter where they left their cell (in the house). These are wireless handsets, and they even announce the contact name or phone number.

If you want it completely separate from your cell, connect an old/cheap device with either a low-priced SIM plan, a google voice account or jmp.chat. Pair that with the Bluetooth home phone for the aesthetic.