Least-cost routing is the process of selecting a telecoms carrier on a trunk provided by another telecoms carrier. It is usually a four-digit code dialled before a number that instructs the telephone exchange to pass the call to another carrier, rather than carry it itself.

For example, you may have lines supplied by British Telecom, but you may want to choose a different carrier because their prices are cheaper. If you don't have a direct connection to the other carrier, you need to inform the British Telecom exchange not to try to connect this call directly, but route it to other carrier network for them to connect. Although you are using a BT line from your premises to the exchange, the other carrier's lines will be used from the exchange to the call's destination. Each carrier has its own access code which, when prepended to the dialled number, the BT line will recognise to which carrier to pass the call over.

A list of such access codes, along with the tariff table you wish to use to cost calls routed by this particular code, is contained in the LCR.CFG file within the main installation folder . 

In the example below, for instance, This configuration would route a dialled number such as  to  (Manchester) via the MCIWorldcom network.

[All LCR Views]

1660 = MCIWorldcom
1452 = Eurobell
132 = Energis