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Least cost routing (LCR) is a method for choosing the route or carrier used to send an outbound telephone call. The system looks at where the call is going, checks which suppliers are allowed to carry it, and picks the one with the lowest cost for that destination. In practice, “least cost” is the optimization goal, not a promise that every call ends up on the cheapest line. Quality limits, capacity, and operator rules can all change the final choice.
How least cost routing works
An LCR engine follows the same basic sequence on every call. The exact order and options depend on the product, but the CGRateS open-source documentation for its LCR feature, shown with version context 0.9.1~rc8, describes the pattern clearly.
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- Match the call to a rule. The engine compares call details against configured routing rules. The documented rule fields include direction, tenant, category, account, subject, destination, rating-profile category, activation time, and weight.
- Filter and rank eligible suppliers. The matched rule names a set of suppliers and a strategy for choosing among them. Eligible suppliers are then ordered or filtered according to that strategy.
- Carry the call on the selected route. The call is sent through the chosen supplier. If that route fails or is excluded, what happens next depends on how the system is configured.
The selection strategies
The CGRateS documentation lists six supplier-selection strategies. Most LCR products offer some version of these building blocks, though names and behavior differ between vendors.
| Strategy | What the selection step does |
|---|---|
| Static supplier list | Uses the supplier order defined in the configuration, without sorting by cost. |
| Lowest-cost sorting | Ranks eligible suppliers from lowest to highest cost for the destination. |
| Highest-cost sorting | Ranks eligible suppliers from highest to lowest cost. |
| QoS filtering | Removes suppliers that do not meet configured quality thresholds, such as average success ratio or call duration. |
| QoS sorting | Orders suppliers by a quality measure rather than by cost. |
| Load distribution | Spreads traffic across the eligible suppliers instead of sending every call to the single top-ranked one. |
Does least cost routing always choose the cheapest route?
No. The cheapest listed rate is only one input. A supplier can be the lowest-priced option for a destination and still be excluded because it fails a quality filter, lacks capacity, or is not eligible under the matched rule. The route that is selected is the best option under the configured policy, which may or may not be the lowest rate on paper.
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Three things commonly separate the lowest listed rate from the route that actually carries the call:
- Quality thresholds. A QoS filter can remove a cheap supplier whose recent answer or completion figures fall below the configured limit.
- Load distribution. Traffic may be deliberately spread across several suppliers, so a call does not always go to the single lowest-cost one.
- Fallback behavior. When the selected route fails, the next route depends on the implementation. Two LCR systems can behave differently here even when both use lowest-cost sorting.
Because of this, a claim that LCR guarantees the cheapest completed call is not supported by the product documentation. The accurate statement is narrower: LCR picks the route that best fits the rules, costs, and quality policies it has been given.
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LCR versus carrier preselection and call-by-call selection
People often use “least cost routing” and “carrier selection” as if they meant the same thing. They do not. The Lebanese Telecommunications Regulatory Authority (TRA), in its SMP Guidelines, defines carrier preselection as a customer’s choice of provider on a permanent or semipermanent basis. It defines carrier selection as a choice made call by call.
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One free scan finds every outdated or missing driver and matches the right update for your exact hardware.Free scan · exact hardware matchBoth are customer-facing choices about which provider carries traffic. An LCR engine is different: it is an automated routing system inside a carrier or enterprise network that applies rules and rates to each call. A customer’s carrier choice can feed into how a system is configured, but the two should not be treated as one mechanism.
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LCR versus interconnection rules
LCR decides which route a call takes inside an operating system. Interconnection rules decide how networks physically and technically connect and exchange traffic. These belong to separate frameworks, and interconnection rules are specific to each jurisdiction and period.
A current example is Mexico. The Comisión Reguladora de Telecomunicaciones (CRT), in its official minimum technical conditions and interconnection tariffs for 2027 to 2029, requires IP-based interconnection and SIP signaling for direct interconnection between covered operators. The regulation states:
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“El protocolo de señalización SIP-IP será obligatorio para la interconexión directa entre concesionarios y de acuerdo con la Recomendación IETF RFC 3261(16) y recomendaciones complementarias.”
In English, this says that the SIP-IP signaling protocol is mandatory for direct interconnection between concession holders, in line with IETF RFC 3261 and complementary recommendations. This applies to Mexico’s regulated interconnection context. It does not describe LCR in general, and it is not a template for other countries.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Where LCR is implemented
LCR is a configurable software function. The CGRateS project documents LCR rules and strategies as part of its charging and routing platform. Oracle’s Network Charging and Control documentation, for version 15.0.1, includes a chapter on least-cost-routing rules and carriers. Both show LCR as a feature within telecom software. Neither shows that LCR must be a dedicated hardware appliance, and the evidence does not support a specific consumer device for it.
What to check when comparing LCR systems
If you are evaluating two LCR products, these are the questions that separate them in practice:
- How destination rates are matched to calls and how they are kept current.
- Whether the policy is strict lowest-price sorting or allows QoS thresholds and weighted, multi-metric selection.
- Which quality and availability metrics are used, and how they are measured.
- How supplier eligibility, capacity, load distribution, and fallback are handled.
- What call records, rating data, and audit trails show about the route that was chosen.
These comparison points reflect what the documented systems allow you to configure. They are not universal features that every LCR product must have.
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Avoid confusing the method with a company name
“Least Cost Routing” also appears as a company name. A Spanish regulator, the CNMC, holds a 2010 case record (RO 2010/253) involving a company called Least Cost Routing Telecom, S.L. and Telefónica. That record concerns the company and its dispute, not the routing method. When you see the phrase in a regulatory or business context, check whether it refers to the technique or to a named firm.
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