Previous systems use relays in the incoming trunk circuit to control ringing and to return busy tone. 5XB uses a "ringing selection switch" (RSS): a crossbar switch with ten verticals, serving ten trunks. The various levels provide various tones, and ringing current of various durations and cadences (especially valuable for party lines). Levels 0 and 1 are used as discriminating levels to set polarity for selective ringing on tip side or ring side. An especially sensitive wire spring RT relay is used to detect off-hook from a line being rung, release the RSS hold magnet, and engage the shielded supervision relay so reverse battery answer supervision would be returned to the originating end.