Editing Ringdown (section)

==Non-operator use==
[[Image:Jimmy Carter Library and Museum 99.JPG|thumb|Hotline telephone without dial]]
In an application ''not'' involving a telephone operator, a two-point '''automatic ringdown circuit''', or '''ringdown''', has a telephone at each end. When the telephone at one end goes off-hook, the phone at the other end instantly rings. No dialing is involved and therefore telephone sets without dials are sometimes used.

Many ringdown circuits work in both directions. In some cases a circuit is designed to work in one direction only. That is, going off-hook at one end (end A) rings the other (end B). Going off-hook at end B has no effect at end A.

Ringdown features are often part of a [[key telephone system]].  In the [[wire spring relay]] key service units of the [[Bell System]] [[1A2]], a model 216 automatic ringdown was used to operate the circuit. In the 400-series units, a number of different [[Key Telephone Unit|KTU]]s operate (supervise) a ringdown, including the model 415. In other situations, the ringdown is powered and operated by equipment inside the [[telephone exchange]].

In the case of enterprises with a [[private branch exchange]] (PBX) switch, the ringdown can be operated by the PBX key. The switch is programmed to ring a specific extension (the called phone) when a defined extension (the calling phone) goes off-hook. The PBX does not offer dial tone to the calling extension: it only detects [[on-hook]] or [[off-hook]] status.

[[Voice over IP]] [[analog telephone adapter|adapters]] can be networked and configured to provide automatic ringdown by selecting a dial plan which replaces the empty string with a predefined number or [[Session Initiation Protocol|SIP address]], dialed immediately. (Some Cisco VoIP phones and analog adapters treat a dial plan of '''(S0 <:1234567890>)''' as a [[hotline]] configuration which dials 1-234-567890 zero seconds after the telephone is taken off-hook, for instance).<ref>[http://www.cisco.com/en/US/products/ps10033/products_qanda_item09186a0080a35977.shtml Getting to Know Dial Plans Sequences]: Document ID #108719, Cisco, Dec 12, 2008</ref>

These circuits are used:

* over high-volume routes where one site calls another very frequently.
:''Example: an information desk and the information desk staff supervisor's desk.''

* where a tamper-proof ability to call from one point to another is needed.
:''Example: a phone used to summon a taxicab to an airport or hotel.''

* where a limited ability to contact one entity (but no ability to make outside calls) is desired.
:''Example: a "house phone" in a hotel lobby to the live operator at the hotel's switchboard''

* where the public, or users that are not trained in using a specific office telephone system, must place calls.
:''Example: the after-hours phone to reach the watchman from the front door at a warehouse.''

* in locations where emergencies are handled and the time required to dial digits would cause an unacceptable delay in handling of an emergency.
:''Example: an airport [[control tower]] to the airport's fire station or fire dispatch center.''
:''Example: [[Independent System Operator]] (ISO) communication to a power plant.''

* in situations where the called party needs to be certain of who is calling.
:''Example: a hospital emergency department and an ambulance dispatch center.''

In some cases, automatic ringdown circuits have one-to-many configurations. When one phone goes off-hook, a group of phones is made to ring simultaneously.

In cases where one or both ends of the circuit terminate in a key telephone system, a well designed system will have no hold feature on the ringdown circuit unless supervision provides a [[Calling Party Control]] (CPC) signal.