An address has to be picked up somehow for those who,for whatever reason find that they can't speak .
Police will listen in to a silent call for suspicious noises then act accordingly. You CAN dial 999 without speaking !!
Yes you can, but NOT without involving BT in finding the caller! You said "that call would have appeared on a digitalised screen
showing the area/address rather than having the loud distracting buzzer. The address of the caller would have been recorded and filtered to the police". The only way to have found out who a caller was back then, would have been to contact BT and have the number traced. Which mean BT would have been involved in the case - they weren't other than to listen into the off the hook line.