Relay calls from one phone to another Does anyone know of an app (on iOS, Android or any platform) that will allow me to place one phone in a part of my house that has good phone signal, and have it relay all calls to another device using my home's WiFi?
Yes - handsfree bluetooth accessories will work to extend the range a good 25 or more feet in many cases. You could also rig a wire and have much, much longer extension, but I can't imagine wanting to be wired to the phone and using a handsfree in that manner.

There are many, many mobile base stations that work with all manner of cellular towers and technologies, so those would be my next choice if you can't get the phone close enough to reception for bluetooth to extend the range to your body.