Permanent signal can also describe the state of a trunk that is seized but has not been dialed upon, if it remains in a busy condition (sometimes alerting with reorder).
When flooded cables or other conditions made this a real problem, switch staff would open the cable, or paper the off-normal contacts of the crossbar switch, or block the line relay from operating.
These methods had the disadvantage of blocking all outgoing calls from that line until it was manually cleared.
Manufacturers also sold devices that monitored the talk wires and held up the cutoff relay or crossbar hold magnet until the condition cleared.
Depending on software version, a wetlisted line may also be in PLO or lockout state.