During the 70s, I was part of a Spiritualist group that was trying to build just such a communication device - their mediums were in communication with several scientists and engineers "on the other side" (i.e. dead, very) who were working on the project. They would give us all the details needed to enable us to build a machine.
I was with the group for about six months; in that whole time, although they spent a lot of time talking to us about it, those dead scientists couldn't manage to communicate even the simplest idea for how such a machine might work. As I remember, the head medium put it down to the fact that nobody in the earthly group had any idea about how electronics worked, so
obviously they couldn't understand the scientists.