Least cost routing (LCR) means your PBX picks the cheapest suitable line for every outgoing call, automatically. Staff dial the number as usual, and the PBX decides whether it goes out on the main SIP trunk, a second operator, a PRI line or a branch line. In India, where many businesses now have two or more operators, a little routing planning can cut the phone bill and also keep calls flowing when one line is down. This guide explains how least cost routing works, how to build it on an IP PBX, and what Indian rules mean for it.
What least cost routing does
Without LCR, every outgoing call takes the same line. With LCR, the PBX looks at the number dialled and chooses a route:
- By number type: mobile, local landline, STD, toll-free, international.
- By operator plan: send calls to the trunk whose plan is cheapest for that kind of call.
- By time: use one route in office hours and another at night, or block a route after hours.
- By capacity: if the cheapest trunk is full, overflow to the next one instead of giving a busy tone.
LCR is not only about cost. The same rules give you failover, because the PBX tries the next trunk when the first is busy or down.
How LCR works in Voxdial IP PBX
Voxdial IP PBX builds least cost routing from two pieces: outbound routes and trunks.
Outbound routes
An outbound route decides which trunk carries a call, based on the digits dialled. Each route has:
- Dial patterns with prefix, prepend and match rules. A pattern can strip the “0” or “9” a user dials for an outside line, or add digits a carrier needs.
- A trunk sequence: an ordered list of trunks to try for that route.
- A route position: routes are checked from the top and the first match wins, so specific routes go above general ones.
- A time group (optional) that limits when the route can be used.
- A route caller ID, route password (PIN) and call recording option.
Trunks
Each trunk has settings that matter for LCR:
- Maximum channels: the most calls the trunk may carry at once. Set it to match what you pay for, so extra calls move on instead of failing at the operator.
- Continue if busy: whether the PBX should try the next trunk when this one is full.
- Outbound dial prefix and number rules: reshape the number into the format each operator expects.
- Disable trunk: take a trunk out of service without deleting it, for example during an operator outage.
A sample LCR plan for an Indian office
Assume a company has a SIP trunk from operator A with a plan that is good for mobile calls, a SIP trunk from operator B that is better for STD landlines, and an old PRI line kept as backup.
| Route (top to bottom) | Matches | Trunk sequence | Notes |
|---|---|---|---|
| Emergency | 112 and other emergency numbers | PRI, then operator A | Marked as an emergency route; always allowed |
| Toll-free | 1800 numbers | Operator A, then B | Free calls on any working trunk |
| Mobile | 10-digit mobile numbers | Operator A, then B, then PRI | A’s plan is cheapest for mobiles |
| STD landline | Numbers with an STD code | Operator B, then A, then PRI | B’s plan is cheapest for landlines |
| International | International prefix | Operator A only | PIN required, blocked after office hours by time group |
Staff do not need to know any of this. They dial the number; the PBX does the rest. When operator A has an outage, mobile calls move to operator B on their own.
Who can use which route
LCR works best together with call restrictions. In Voxdial, class of service rules decide which extensions may use which routes. A lobby phone may only reach internal and emergency numbers, sales may call mobiles and landlines, and only a few managers may dial international numbers. See how to control your office phone bill with call restrictions for practical plans.
Indian rules to keep in mind
LCR must stay within Indian telecom licensing rules. In general:
- Each call should go out on a line in the same service area as the caller. Sending a PSTN call through a trunk in another city to save STD charges, known as toll bypass, is not permitted for ordinary enterprises.
- Calls between your own branches over your private network are fine as internal extension calls, as long as they do not connect to the public network at the far end to avoid charges.
- Caller ID must be a number allotted to you on the trunk the call uses. If a call can overflow to a second operator, set a valid number from that operator too. See outbound caller ID.
Rules and plans change, so confirm the details with your operators when you design the routes.
Steps to set up least cost routing
- Collect your plans. For each trunk, note the per-minute or bundled rates for mobiles, local, STD and international, plus the number of channels.
- Group numbers by type. Decide which dial patterns represent each group of numbers.
- Rank trunks for each group. Cheapest first, then the backup lines.
- Build the routes. Specific routes such as emergency and toll-free go at the top, general ones at the bottom.
- Set maximum channels and continue if busy on every trunk, so overflow works.
- Add class of service and time groups for international and premium numbers.
- Test every route. Make a test call of each type and check the operator’s call records to confirm it went out on the expected trunk.
- Review monthly. Compare operator bills with the outgoing calls in the PBX call reports, and adjust the routes when plans change.
The PBX dial plan guide covers how to choose the outside-line digit and keep codes from clashing, and PBX backup and failover covers trunk failover in more depth.
Want least cost routing designed around your operators and branches? Talk to BitVoice.
FAQs
What is least cost routing in a PBX?
Least cost routing is a set of rules that makes the PBX choose the cheapest suitable trunk for each outgoing call, based on the number dialled, the time and which trunks are free.
Does least cost routing need two operators?
It helps most with two or more trunks, but even one SIP trunk plus a PRI or analog line gives you a cheaper first choice and a backup.
What happens if the cheapest trunk is busy?
If the trunk has continue if busy turned on, the PBX tries the next trunk in the route’s sequence, so the call still goes out.
Is routing calls through another city’s trunk allowed in India?
Generally no. Sending public calls through a trunk in another service area to avoid charges is toll bypass. Keep each call on a line in the caller’s own area, and check with your operator.
