page_title: Foreign exchange service (telecommunications)
section_title: Function
hierarchical_section_title: Foreign exchange service (telecommunications) / Function
caption_attribution_description: English: Hungarian Telephone Factory - 1937. Budapest
context_page_description: Foreign exchange service is an access service in a telecommunications network in which a telephone in a given exchange area is connected, via a private line, as opposed to a switched line, to a telephone exchange or central office in another exchange area, called the foreign exchange, rather than the local exchange area where the subscriber station equipment is located.
  To call originators, it appears that the called party having the FX service is located in the foreign exchange area. It is assigned a telephone number of the foreign exchange.
  The telecommunication circuit between central offices that implements foreign exchange service has complementary interface types at each end. At the foreign central office that provides the service, the interface is called the foreign exchange office end, and at the end where the subscriber station is connected, it provides the foreign exchange station interface.
context_section_description: An FX line has the local calling area of the foreign exchange in which it is numbered.
  A subscriber located just outside the exchange boundary of a large city, or just outside the flat-rate local calling area for the city, would find that many numbers which would have been local from the city itself became long-distance. In many areas, local flat-rate service was subsidized by long-distance toll service for much of the 20th century. As an "FX line" has a number from the neighboring city, it has the city calling area for both incoming and outbound calls.
  For instance, a suburban business may want to market extensively to Toronto, a large city with flat-rate local calling:
  If the business is in an adjacent suburb (such as Mississauga or Markham) a local number will reach the city but not the suburbs on the other side. Adding an FX line with a Toronto +1-416 number would provide full coverage.
  If the business is located just outside the larger city's local calling area, an FX number in the next-closer suburb would provide a limited coverage of the city. An Oshawa business may lease an FX line from suburban Ajax as that community is local to both Toronto and Oshawa, even though Ajax does not have the full Toronto calling area.
  The "FX line" is usually treated as part of the distant city when originating calls to N11-style numbers, such as information or emergency telephone numbers.
  While a cost of hundreds of dollars monthly for the leased line was not uncommon, to a business handling large volumes of calls from the larger city the cost may have been justified by long-distance toll savings at a time when long-distance was pricey and alternatives were limited.
  Originally, the FX line was a physical copper pair of telephone wires from the foreign exchange which were connected to the local subscriber loop at the local exchange, without passing through the local switch. This dedicated circuit is often replaced with a virtual circuit, where the local switch sends the FX calls to the foreign exchange (which handles all billing) on existing trunks.
  In rare instances, the supposed "foreign" exchange actually resided on the same physical telephone exchange at the same location, but clients were billed based on nominal centre-to-centre distance between different rate centres.
  A similar "FCO" service provided no difference in local calling area (the distant exchange is in the same rate centre). Historically, it was a means to obtain features not available on the local exchange (such as DTMF tone dialling when first introduced in 1963) or keep an existing business telephone number operational after a cross-town move.
  Conventional "foreign exchange" leased lines and their variants have become less common due to newer alternatives:
  An outbound "extender" is an automated local number at a service bureau in the larger city. A suburban subscriber (who can call the city itself locally but is long distance to suburbs on the other side) could call the extender locally, get a city dial tone and dial back out locally to the larger area.
  Remote call forwarding served a similar function for inbound calls only. A suburban business could get a downtown big-city number; clients anywhere in the larger city's coverage area could call locally, only to be silently redirected via a second local call to the destination.
  Interactive voice response systems have been hosted at answering service bureaux for clients such as suburban radio stations  accepting calls from listeners in the larger city. As the machine is on a city number, it is reachable from the full metropolitan calling area.
  Mobile telephone exchanges (in countries which use geographic numbers) normally are issued from the larger city and have that city's full calling area.
  Voice over IP numbers may be obtained from most cities and used almost anywhere in the world. VoIP renders the subscriber's physical location meaningless, as long as unrestricted broadband Internet is available at the site. Local number portability allows an existing number to be moved to VoIP (or, in some countries, a mobile t
caption: A foreign exchange office (FXO) device plays the role of an analog phone
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