I made this 3 months ago but forgot about it.
After watching Proving Demons video about the spirit phone (you can watch it here: https://www.youtube.com/watch?v=pNwKH3IFHeg) I thought I would try and localise the feedback using bluetooth.
Instead of creating a huge radio loop using mobile to a cellular tower and them over to the other phone, it could be localised at the exact location of the phone.
Just an idea.