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Cost Attribution for Communication Spend: Who Is Using the Phone System
Most organisations know their total telephony spend to within a few percent and know almost nothing about which department or site generates it. That is enough to manage the bill and not enough to make decisions. When a business needs to know whether a department is genuinely high-volume or merely badly configured, when a site is overspending on trunk channels, or when a shared cost needs allocating fairly, attribution becomes necessary.
The Three Levels of Attribution
| Level | What it attributes | Requires | Use it for |
|---|---|---|---|
| Trunk level | Cost to a trunk, which maps to a site or carrier service | Trunks mapped to sites | Site cost control and channel sizing |
| Extension level | Cost to individual extensions or departments | Extension to department mapping | Department cost allocation |
| Cost centre level | Cost to a cost centre including its shared allocations | Shared cost rules agreed with finance | Chargeback and reporting |
Mapping Extensions to Cost Centres
Extension to department mapping is the practical foundation, and it should be maintained rather than assumed. When an extension moves to a different department, the mapping changes and the historical data becomes ambiguous. Maintaining the mapping so that it reflects the period the calls were placed, rather than the current state, is what makes historical cost reporting reliable.
Allocating Shared Costs Honestly
A significant portion of telephony cost is shared: main numbers, central trunks, common licenses, and platform capacity. There is no single correct allocation, and pretending otherwise damages trust. What matters is that the allocation basis is chosen, documented, applied consistently, and understood by the departments it affects. Common bases include extension count, direct cost, or headcount, and each answers a slightly different question.
| Allocation basis | Sensible when | Distorts when |
|---|---|---|
| Extension count | Cost is largely driven by user count | A few users make very long or high-volume calls |
| Direct call cost | The goal is to attribute variable cost fairly | Ignores shared capacity consumed by occasional heavy users |
| Headcount | Cost should broadly follow organisational size | Departments with unusual call intensity |
| Hybrid, reviewed periodically | The estate is complex | Requires maintenance discipline to stay fair |
Cost Per User Without Misleading Averages
Cost per user is the most commonly misused figure. A department with ten staff making a few calls each has a high cost per user. A department with three staff running a contact centre has a low one. Neither is wrong; they are different organisations. Report cost per user alongside cost per call and the call volume distribution, so the average is not read as a performance measure of the department.
Using Attribution to Make Decisions
- Identify channels that are unusually expensive — High cost on low usage often indicates misrouting or toll fraud
- Right-size trunk channels by site — Sites paying for channels they never use at peak
- Identify unlicensed or unmanaged extensions — Cost appearing where no mapping exists
- Test chargeback impact before implementing — Departments respond strongly to cost they can see
- Review allocation rules annually — Organisations and call patterns change
Redstone Approach
Redstone IP-PBX platforms produce call detail records including trunk, extension, cost, and call outcome, and support scheduled export so that cost attribution can be performed outside the platform and reviewed with finance. Redstone can help map extensions to cost centres and design the reporting that supports an agreed allocation basis.
Redstone supports telephony cost attribution with call detail export, extension to cost centre mapping, and reporting design aligned with finance's allocation requirements.
Redstone Systems, Inc. founded in Delaware, USA in December 2002, has been the ODM vendor for many well-known communications companies, serving the Southeast Asian market. In 2020, Redstone Systems will begin to return to the North America market with its self-developed brand.
Redstone has a complete product line of intelligent voice gateways, providing IP-PBXs, analog VoIP gateways (FXS/FXO), digital VoIP gateways (E1/T1), border appliances, and session boundary controllers (SBCs).
With advanced technology in digital signal processor (DSP), speech coding and speech processing, as well as efficient operational tools such as cloud remote management, auto provisioning, Redstone gateways are widely used in markets of enterprise communications, cloud communications, call centers, operators’ IMS/SIP trunks, bringing users friendly, efficient and reliable communication experience.
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