Chevron,
I can remember reading a book about Humberside Airport. There was quite a detailed ATC section and it mentioned something about the presence of PBXs at six regional locations. So I guess this is where the Mediator lines all "met". I don't know if this is still the case. When the tower at EGNM had the old RDCE gear external phone lines were accessed using old BT-style Autodial consoles, basically grey plastic boxes with yellow buttons. Sadly long since cast in the skips !
ZOOKER. Check your PMs please. Ta !