You've got to seize the line circuit which routes you to a Group selector. That operates the A relay that operates the B and C relays. This allows dial tone from a motorized generator to be induced into the line via the A relay coils. Dialling, or tapping the cradle, flaps the A armature, which has notches to lighten it's mass, which drives the Group selector vertical the same as dialled. It then hunts horizontally to find another available group and the sequence repeats for the second number dialled. And so on. Gawd I loved those days! Walking into a silent building full of racks of equipment and leaving behind a fully functional, and very noisy, telephone exchange.
<sigh> The good old days! Now that it's pretty much all electronic, pulse dialling is no more.
Edit for correctness: That B relay was slow to release and stayed operated while the A did flap!