PBX Sizing Guide: How Many Lines Does Your Business Actually Need?
Overbuying costs money every month. Underbuying collapses call flow. This guide gives you the formula to get it right the first time.
The short answer
Most South African businesses need one concurrent call path for every 3–5 staff who make or receive calls. A 20-person team typically needs 4–7 simultaneous lines. Cloud PBX makes this easy to adjust over time, but you still need a baseline number before you buy; otherwise you end up paying for capacity you will never use.
users per concurrent line (light-use baseline)
users per line (call-centre or heavy-call environments)
% buffer to add on top of your calculated peak
The sizing formula
PBX sizing comes down to one number: peak concurrent calls. That is the maximum number of calls your business could be handling at the same exact moment during your busiest hour. Every other number (extensions, users, handsets) is secondary to this figure.
Peak concurrent calls = (Number of staff who make/receive calls) ÷ (Users-per-line ratio for your call volume)
Then add a 20% buffer for growth and peak spikes.
Choosing your ratio
The users-per-line ratio depends on how often your team is actually on a call:
| Call environment | Typical ratio | Example: 30 staff |
|---|---|---|
| Light use (email-first, occasional calls) | 5:1 – 6:1 | 5–6 lines |
| Mixed use (sales, support, admin blend) | 3:1 – 4:1 | 8–10 lines |
| Heavy use (inbound call centre, outbound sales) | 2:1 – 3:1 | 10–15 lines |
If you are unsure, start at 4:1, monitor your first month’s call logs, and adjust. Cloud PBX makes scaling up or down a settings change, not a hardware order.
Sizing by business size
Use this table as a starting point. The ranges account for both light-use and heavy-use scenarios within each headcount band.
| Business size | Staff on phones | Concurrent lines needed | Extensions to provision | Recommended setup |
|---|---|---|---|---|
| Micro (1–10 staff) | All | 2–5 | 10–15 | Entry Cloud PBX plan; single hosted instance |
| Small (11–50 staff) | 70–80% | 5–20 | 20–60 | Cloud PBX with IVR, call queues, and mobile app |
| Medium (51–200 staff) | 60–75% | 15–60 | 60–250 | Multi-site Cloud PBX with call recording and reporting |
| Enterprise (200+ staff) | 50–70% | 50–200+ | 200+ | Cloud or hybrid PBX; dedicated voice infrastructure |
Important: provision extensions at 1.2–1.5× your current headcount. Extensions are cheap to add in Cloud PBX; running out of extension capacity mid-growth causes operational disruption.
Which sizing mistake are you making?
Most businesses make one of two errors. Identifying yours early saves time during the quoting process.
Cloud, on-premise or hybrid: which hosting model fits your size?
Concurrent line count is the primary variable. Hosting model is secondary, but it affects upfront cost, control, and maintenance overhead significantly.
Cloud PBX
Best for: micro to large businesses (1–200+ staff)
The provider hosts the PBX in a data centre. You provision extensions through a web portal; lines scale with a settings change. No hardware investment, no IT overhead. Monthly per-seat pricing.
Sizing advantage: you can add or remove concurrent lines within the same billing cycle. Ideal if your call volume is uneven or growing.
On-premise PBX
Best for: businesses with fixed headcount and consistent call volumes (typically 50+ staff)
Hardware lives in your server room. You buy or lease the physical appliance and licence blocks. Upfront cost is higher; scaling requires purchasing additional hardware or licence packs.
Sizing consequence: once you buy a hardware PBX for N lines, upgrading beyond that capacity means buying a new chassis or expansion card. Size conservatively but account for 2–3 year growth upfront.
Hybrid PBX
Best for: businesses with an existing on-premise investment that need cloud flexibility for mobile or remote staff
A soft-client or SIP gateway connects on-premise extensions to cloud trunks or a hosted SBC. On-site staff use desk phones; remote staff use softphones or mobile apps.
Sizing note: count remote workers separately in your concurrent line calculation; they consume cloud trunks even when on-site lines are idle.
5 steps to size your phone system correctly
- Count your calling staff. Exclude staff who never make or receive external calls (warehouse floor workers, delivery drivers with separate mobile allocations, etc.). This is your base population.
- Identify your call environment. Are you mostly email-first with occasional calls (5:1 ratio), or is your team on the phone constantly (2:1)? Pull your last phone bill or call log if you have one; peak-hour call counts are the most accurate input.
- Apply the formula. Divide calling staff by your ratio. If you have 40 calling staff in a 4:1 environment: 40 ÷ 4 = 10 concurrent lines needed.
- Add the 20% buffer. 10 lines × 1.2 = 12 lines. This covers seasonal spikes, staff growth, and occasional simultaneous call bursts. In Cloud PBX, the cost of this buffer is typically a few hundred rand per month, small compared to the cost of dropped calls.
- Review after 90 days. Your first 3 months of live call data will tell you whether you over- or under-sized. Cloud PBX makes adjustments straightforward; set a calendar reminder to pull the concurrent-call peak report and dial the number up or down.
“The most common mistake is quoting extensions, not lines. A business with 50 desk phones does not need 50 simultaneous call paths, but they do need the right number based on actual calling behaviour.”
WhichVoIP editorial
Verdict
The users-per-line ratio (3:1 for heavy call environments, 5:1 for lighter ones) plus a 20% buffer gives you a defensible starting figure before you request a single quote. Once you are live on Cloud PBX, the concurrent-call peak report is your ongoing calibration tool; review it quarterly and adjust.
If you are buying an on-premise system, size for your 2–3 year projected headcount from the start. On-premise hardware scales in hardware increments, not a settings toggle.
FAQ
What is the difference between extensions and concurrent lines?
An extension is an internal number assigned to a person or device (desk phone, softphone, mobile app). A concurrent line is one live call path at a specific moment. You can have 50 extensions but only 10 simultaneous calls happening at once, meaning you need 10 concurrent lines, not 50. Providers sometimes quote extensions when they should be quoting concurrent line capacity; always confirm which number they mean.
How many phone lines does a 20-person business need?
For a 20-person team in a mixed-use environment (sales, support, admin), apply a 4:1 ratio: 20 ÷ 4 = 5 concurrent lines. Add the 20% buffer to get 6 lines. If your team is heavier on calls (outbound sales, inbound support), use a 3:1 ratio: 20 ÷ 3 = 7, rounded up to 8–9 with the buffer. Start at 6, monitor your first month’s peak concurrent call report, and adjust from there.
Can I add lines to a Cloud PBX later?
Yes: adding or removing concurrent call capacity is one of Cloud PBX’s core advantages over on-premise hardware. Most providers let you adjust your plan through a web portal or a quick request to your account manager. The change typically takes effect within 24–48 hours. Check your provider’s contract for minimum commitment periods; some lock line counts for a fixed term even on “flexible” plans.
Do I need a separate line for each handset?
No. A single SIP trunk (concurrent line) can support multiple registered devices as long as only one call is active at that moment. Each handset gets its own extension number. The number of lines you need is driven by simultaneous active calls, not the number of physical phones or softphone registrations.
Does a call centre need a different sizing approach?
Yes. Call centres use Erlang traffic models (specifically Erlang C for queued inbound calls) to calculate precise agent and line requirements. The ratio method in this guide is accurate enough for general business environments, but a high-volume inbound contact centre should request an Erlang calculation from their provider based on target service levels, average handle time, and expected call arrival rate.
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Sources
- ITU-T G.114 (May 2003): One-way transmission time (150 ms) – baseline for voice latency standards
- Erlang traffic theory: standard telecommunications load calculation method for concurrent call sizing