The analogy with free / busy tone on PSTN is inexact, as the " on-hook " telephone status reflects the ability of the network to reach the recipient after the requester has initiated the conversation.
12.
Previous systems used relays in the incoming trunk circuit to control ringing and to return busy tone . 5XB used a " ringing Selection Switch " ( RSS ), a crossbar switch with ten verticals, serving ten trunks.
13.
This tone was adopted in the mid to late 1960s and replaced the older busy tone, which was the same 400 Hz signal but at half the interruption rate ( i . e . 0.75 s on, 0.75 s off ).
