An early exchange, call setup
nOne of the 100 subscribers lifts his handset -> Call finder is activated to search the line.nAfter the line is located other relays connect the dial-tone generator.
nThe subscriber selects two digits.
nThe first digit selects the subscriber group by using the group selector.
nThe second digit selects  the line selector.
nSelection is done by sending pulses that move the selectors stepwise.
nWhen connection is established a ringing tone is sent.