Using an old iphone as a wifi only device at home (no cellular number)

I have a new iphone and use that as my mobile device.

I am interested in using my old iphone more like a smaller wifi-only ipad.

I have no problem doing that and browsing and using various apps. The problem is with Google Voice. I would like this device that sits by my chair in the living room to be able to make and receive via VOIP (not celllular) just like I can with my ipad and with a computer. I am using Google Voice and it seems to insist on linking a cellular phone number which doesn't exist on this old phone.

Any ideas? Am I stuck using an ipad or ipad mini if I want that Google Voice VOIP connection in my living room?

Thanks

David C