Permanent signal

Permanent Signal (PS) in US telephony jargon, 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. This is indicated in modern switches by the silent termination after the off hook tone times out and the telephone exchange computer puts the telephone line on its High & Wet list or Wetlist. In older switches, however, a Permanent Signal Holding Trunk (PSHT) would play either a howler tone or a 480/500 Hz high tone (which would subsequently bleed into adjacent lines via crosstalk). Howler 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.