Permanent signal (PS) in American

telephony Telephony ( ) is the field of technology involving the development, application, and deployment of telecommunications services for the purpose of electronic transmission of voice, fax, or data, between distant parties. The history of telephony is ...

terminology, or permanent loop in British usage, is a condition in which a POTS line is off-hook without connection for an extended period of time.Telcordia Technologies, GR-505-CORE Issue 2, December 2005, ''Call Processing'', Section 4.5.2 This is indicated in modern switches by the silent termination after the

off-hook tone The off-hook tone (also off-hook warning, howling tone, or howler tone) is a telephony signal for alerting a user that the telephone has been left off-hook without use for an extended period, effectively disabling the telephone line. North Ame ...

times out and the

telephone exchange A telephone exchange, telephone switch, or central office is a central component of a telecommunications system in the public switched telephone network (PSTN) or in large enterprises. It facilitates the establishment of communication circuits ...

computer puts the

telephone line A telephone line or telephone circuit (or just line or circuit industrywide) is a single-user circuit on a telephone communication system. It is designed to reproduce speech of a quality that is understandable. It is the physical wire or oth ...

on its ''High & Wet list'' or ''Wetlist''. In older switches, however, a Permanent Signal Holding Trunk (PSHT) would play either an

(''howler tone'') or a 480/500 Hz high tone (which would subsequently bleed into adjacent lines via

crosstalk In electronics, crosstalk (XT) is a phenomenon by which a signal transmitted on one circuit or channel of a transmission system creates an undesired effect in another circuit or channel. Crosstalk is usually caused by undesired capacitive, ...

). Off-hook tone is a tone of increasing intensity that is intended to alert telephone users to the fact that the receiver has been left off the hook without being connected in a call. On some systems before the off-hook tone is played, an

intercept message An intercept message is a telephone recording informing the caller that the call cannot be completed, for any of a number of reasons ranging from local congestion, to disconnection of the destination phone, number dial errors or network trouble a ...

may be announced. The most common message reads as follows; "If you'd like to make a call, please hang up and try again. If you need help, hang up and then dial your operator." 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 mid-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 In electronics and telecommunications, a crossbar switch (cross-point switch, matrix switch) is a collection of switches arranged in a Matrix (mathematics), matrix configuration. A crossbar switch has multiple input and output lines that form a ...

, 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.

References

Local loop Telephony signals {{telephony-stub