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).
In most middle 20th century switching equipment, a permanent signal would tie up a junctor circuit, diminishing the ability of the switch to handle outgoing calls. 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. Some crossbar line circuit designs had a park condition allowing the line circuit itself to monitor the line.
Stored program control exchanges finally solved the problem, by setting a bit mask in the scanning program and running a low priority periodic checking program against the wetlist. Depending on software version, a wetlisted line may also be in PLO or lockout state.