Foreign exchange service (telecommunications)

Foreign exchange service (FX) is a telecommunications network service in which a telephone in a given exchange area is connected, via a private line (as opposed to a trunk line), to a telephone exchange or central office in another foreign exchange, rather than the local exchange area where the device is located.

To call originators, it appears that the called party having the FX service is located in the foreign exchange area.

In basic telephony there are two types of offices: local and foreign. A local office was assigned a specific area, and all telephone services provided to that area came from that central office. Each central office had its unique identifier. In the early days names were used, such as "Jackson" or "Newton". The office names were changed to three-digit numerical exchange codes (NNX), prefixed to the local phone number (not the area code).

Customers who wanted a telephone number provided by a neighbouring telephone central office leased a "foreign exchange" line. With the old two-wire loop technology, this would require an engineered circuit with increased costs. The practice, rare except in big cities, is in decline.

Foreign Central Office (FCO) or Foreign Zone (FZ) were, from a technological standpoint, deployed with the same methods as Foreign eXchange (FX). They differ only in that the remote office is in exactly the same rate centre (FCO) or merely in a different zone of the same US metropolitan city (FZ). Much like FX service rates depend on the distance between rate centres, FCO service prices depend on the distance between exchanges.

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 subsidised by artificially-expensive long-distance toll service for much of the 20th century. As an "FX line" is a number from the neighbouring city, it has the full big-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:

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