Perchance the Panic Button wire was severed, would alarm have automatically, by default, been alerted at EP Control?
I have no information about how police-supplied panic buttons work. A typical burglar alarm of the period would consist of more than just a panic button, but could include one or more panic buttons. Those buttons would not normally be hidden. They were wired in and disconnecting the wiring would activate the alarm. However, such an alarm system, if "monitored", dialled out to notify the monitoring centre, which wouldn't be possible while any telephone on the same line was connected and off-hook. It's conceivable, though perhaps unlikely, that police-supplied panic buttons were more sophisticated. Do you remember the very old call boxes with "A" and "B" buttons? They also had a completely separate third button that could be pressed in emergency. I don't know what that button did, but presumably it somehow allowed an emergency call without the person who pressed it needing to dial.