The thing about the phones is puzzling, why was the kitchen one hidden, why take the phone from upstairs and put it in the kitchen when the kitchen one was still in there (albeit hidden). Perhaps the benefit to the killer was that the old style phone was exactly that, older in technology terms than arguably the other phones. The questions abound. Maybe Jeremy could clarify?
As for the internal intercom systems, as far as I know, only one line was needed, no idea how they worked though. The phone may have nothing to do with the main issues. For example, we dont know whether RB was shot first downstairs or first upstairs, he could have been dragged by the killer either way or made his own way (albeit heavily injured) up or down the stairs.
Feel free to have a look on
www.sleuthingforjustice.com where Mike Tesko spent a long time posting before coming to this site.