Least cost routing (LCR) is the method a telephone system uses to choose which carrier or route carries an outbound call, based on what it costs to reach that destination and on rules the operator has configured. The name describes the optimization goal, not a guarantee. Most LCR systems also filter suppliers by quality, eligibility, or capacity before the cheapest remaining option wins, so the lowest listed rate does not always produce the route the call actually uses.
What least cost routing means
LCR is route selection inside an operating system, usually a voice switch, softswitch, or billing and rating platform. For each outbound call, the system compares the call’s details against a set of routing rules, identifies the suppliers (carriers or trunks) that are allowed to carry that call, ranks them, and sends the call on the selected route. The cost figures come from rate tables that map destinations to prices, so the accuracy of an LCR decision depends on how well those tables are maintained.
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How an LCR decision is made
- Match the call to a rule. In the CGRateS LCR system (documented under version context 0.9.1~rc8), a rule is matched using fields such as direction, tenant, category, account, subject, destination, rating-profile category, activation time, and weight. Which fields an operator uses depends on how the platform is set up.
- Filter eligible suppliers. The candidate suppliers for the matched rule are reduced to those allowed to carry the call. Quality filters, where configured, can remove suppliers at this step.
- Sort or distribute. The remaining suppliers are ordered by the configured strategy, such as ascending cost, or spread across suppliers by load distribution. The strategies are listed in the next section.
- Carry the call. The call is sent on the selected route. What happens if that route fails, and whether the system tries the next supplier in order, depends on the implementation and its configuration. Two LCR products can behave differently at this step even when their rate tables are identical.
The selection strategies
CGRateS documents a set of supplier-selection strategies. The table below describes each one and what it means for the route a call takes.
| Strategy | What it does | What it means for the outcome |
|---|---|---|
| Static supplier list | Uses a fixed supplier order set by the operator. | Prices are not compared per call; the order is whatever was configured. |
| Lowest-cost sorting | Ranks eligible suppliers from cheapest to most expensive. | The cheapest eligible supplier is selected first. Quality is ignored unless a quality filter or sort is also applied. |
| Highest-cost sorting | Ranks eligible suppliers from most expensive to cheapest. | The most expensive eligible supplier is selected first, which is the reverse of least-cost sorting. |
| QoS filtering | Excludes suppliers that fail quality thresholds. Documented measures include average success ratio and call duration. | A supplier below the threshold is removed from the candidate set, even if it is the cheapest. |
| QoS sorting | Ranks suppliers by quality measures. | Quality, not price, determines the order. |
| Load distribution | Spreads calls across suppliers. | Calls are shared rather than all sent to the single cheapest route. The distribution method depends on configuration. |
These strategies are documented as configurable options in CGRateS. They are not a fixed list that every LCR product must offer, and the exact labels and combinations vary by platform.
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Does least cost routing always choose the cheapest route?
No. The phrase describes the sorting objective, and a configured LCR system can override strict price ordering in several ways:
- Quality thresholds. A supplier with the lowest rate can be excluded if its success ratio or call duration falls below the configured limit.
- Eligibility and capacity. A supplier that cannot take the call, or is marked unavailable for that destination, is not a candidate.
- Load distribution. A deliberately shared load sends some calls to suppliers that are not the cheapest.
- Fallback behavior. If the chosen route fails, the next route in the configured order may carry the call at a higher rate.
The practical distinction is between the lowest listed rate and the route that meets the operator’s eligibility and quality policy. An LCR engine can be correct to a rule while the completed call still costs more than a theoretical minimum, because the rule weighed something other than price.
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LCR is not carrier preselection
Readers often conflate LCR with the consumer choice of carrier. The Lebanese Telecommunications Regulatory Authority (TRA), in its SMP Guidelines, separates two customer-side concepts. Carrier preselection is a customer’s choice of provider on a permanent or semipermanent basis. Carrier selection is a choice made call by call. Both describe who chooses the carrier and how long that choice stands.
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Outbyte PC Repair FREEClear out junk files and repair common Windows errorsFree Scan →Outbyte Driver Updater FREEFix the driver behind crashes, sound loss and screen glitchesFind Drivers →An LCR engine is different in kind. It is an operator or enterprise system applying its own rules to each call, usually based on cost and policy rather than a subscriber’s preference. The TRA terms help explain the customer side of carrier choice; they do not describe how an automated routing engine works.
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- Computing and enabling technologies associated with modern telecommunications
- LANs, data centers, and VoIP PBXs
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- Broadband and wide area networks
LCR is not the same as interconnection rules
Interconnection rules specify how networks physically and technically connect and exchange traffic. They are set by regulators or negotiated between operators, and they are specific to a jurisdiction and a time period. LCR sits on top of this: it decides which available route to use, but it does not define how those routes must be built.
Mexico offers a current example. The Comisión Reguladora de Telecomunicaciones (CRT) published official minimum technical conditions and interconnection tariffs for 2027 to 2029, through the Diario Oficial de la Federación. The provisions cover IP interconnection, redundancy, Ethernet, SIP signaling, and related arrangements. On signaling, the official text states:
Rank #4
“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, the provision means 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 and to the operators the regulation covers. It is not a template for other countries, and it does not define how any LCR installation must be configured. The technical quantities in the regulation, such as link and port capacities, describe interconnection requirements; they are not measures of how well LCR works.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Where LCR runs
LCR is delivered as configurable software. The CGRateS project, an open-source rating and charging engine, documents LCR rules and supplier-selection strategies. Oracle Communications Network Charging and Control, in its documentation at document version path 15.0.1, includes a chapter on least-cost-routing rules and carriers. Both show LCR as a function of platform software. Neither establishes that LCR must run on a dedicated appliance, and neither addresses consumer equipment.
Questions to ask when comparing two LCR systems
The documented strategies and rule parameters suggest a practical set of comparison points. They are not universal requirements, but they show where two products can produce different routes from the same rates:
- How rates are matched to destinations and how they are kept current.
- Whether the policy is strict lowest-price sorting, or whether it allows quality thresholds and multi-metric selection.
- Which quality and availability metrics are used, and how they are measured.
- How supplier eligibility, capacity, load distribution, and fallback behavior are handled.
- What call records, rating, and audit data show about the route that was selected.
Do not confuse the method with a company name
“Least Cost Routing” also appears as a company name. A Spanish regulatory case recorded by the CNMC under RO 2010/253 involved Least Cost Routing Telecom, S.L. and Telefónica. That record concerns a named company in a historical dispute, not the general routing method described here.
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