Prior to modern-day Zeatron dispatching, each station would have a device called a joker.  The Joker would punch holes in a moving tape roll.  So, one hole, pause, two holes would correspond to Box 12.

X    X X

One hole, pause, two holes, pause six holes would be Box 126.

X    X X    X X X X X X

The system was designed to repeat the Box number three times, in case you missed it the first time. It was the responsibility of the house Watchman to look up the box numbers and announce which units from that house were to respond.