What is LCR and How does it Work?
LCR is the technique that allows selecting what route to use based on criteria; usually the price. The idea behind the LCR is not only saving money and maximize profits but to have redundancy as well. Many PBX's offer an LCR mechanism as part of the software.
How does LCR Work?
It is very common that many carriers offer the same route to the same place. It is common that the price they offer is different, but looking closely into their rate sheets it is easy to find that for Carrier A has a cheaper price to connect to destination X, and Carrier B has a cheaper price to connect to destination Y. Imagine now having more than three carriers, route optimization wouldn't be possible without LCR.
LCR as a Mechanism for Fault Tolerance and High Availability
LCR also gives you redundancy. When an LCR mechanism is requested for the route to destination X, it returns a list of routes ordered by your selected criteria (price for example). If the best option fails to connect, the LCR will tell the PBX routing mechanism to use the second-best option and so on.
LCR is not just a way to use always the cheapest rate. Many LCR systems will have loaded at least two carriers. This means, that if the first carrier fails to route, the LCR system will try the second-best option and so on until one route hits or it runs out of options.
You must know that depending on your PBX configuration, this may carry and uncomfortable side-effect. The end-user may experience a dead-air while the LCR system is trying to find an option for the route. The easiest workaround for this is sending a 183 SIP answer with the proper ring-back tone, this way your end-user will listen to something.