Most small businesses need far fewer lines than they have staff. A phone system for small business is sized on simultaneous calls. That is the number of conversations running at the same moment. For an ordinary small office it is a fraction of the headcount. You settle it by measuring your busiest hour.
You may have a quote in front of you, or a provider asking how many lines you want. The number in your head probably came from counting desks. That is the copper answer, and it is worth checking.
A phone line was a physical pair of copper wires that carried one call at a time. That is where the habit of buying one line per desk comes from.
Its modern equivalent is a channel, often called a SIP channel. SIP is the standard that carries calls over an internet connection instead of copper. A channel still carries one call at a time, so the old idea survives without the physical pair. Small business telephone systems have been sold by the channel for years.
nbn’s switch-off page says most existing fixed line phone and internet services are being replaced by services on the nbn network. Some are not affected, including those on non-nbn fibre networks, some in apartment complexes, and some business and Special Services.
An extension is the internal number that reaches one person or one handset. A phone number is an address the outside world dials.
Neither is a line. 3CX’s published table expects forty extensions to run on eight simultaneous calls. A business can give every person a direct number and still buy a handful of channels. The number is the address. The channel is the path.
A simultaneous call is one conversation in progress, and it is the unit small office phone systems are sized in.
3CX is the platform many Australian small businesses run. It licenses a system by the number of calls happening at once, internal and external. Its pricing page and its November 2025 announcement carry these rows, read 23 September 2026.
| Simultaneous calls | Maximum extensions | Extensions 3CX recommends |
|---|---|---|
| 4 | 15 | 1 to 15 |
| 8 | 40 | 16 to 40 |
| 16 | 80 | 41 to 80 |
Those ranges are 3CX’s own guidance for call quality and availability, not an industry standard, and it changed them in November 2025. The older ranges allowed three to four extensions for each simultaneous call. The current maximums sit at five to eight, which the announcement calls almost double the previous guidance.
3CX counts internal calls as well as external ones. Two staff talking to each other on extensions use a simultaneous call, even though nothing leaves the building. A business with a lot of internal traffic sizes higher than its outside calls suggest.
Cisco’s traffic analysis documentation sets out the measurement that settles it. Traffic is counted in erlangs. One erlang is 3,600 seconds of calls on the same circuit, which is enough to keep one circuit busy for an hour. You measure over the busiest hour, because that is the heaviest load the system carries.
Count the calls in your busiest hour, multiply by the average call length in minutes, then divide by 60. Forty calls at three minutes each is two hours of talking inside one hour. That is two erlangs.
The margin that keeps callers off a busy signal is bigger than instinct suggests. Grade of service is the chance a call is blocked as it reaches for a channel. P.01 means one call in a hundred. Two erlangs needs about seven channels at one in a hundred blocked, or about six at one in fifty. One erlang needs about five, or four at the looser setting. Four erlangs needs about ten.
Those counts come from the Erlang B formula Cisco publishes, at the blocking rates named. Erlang B assumes a blocked call goes away rather than waiting. Cisco’s own example uses the Poisson model, a more cautious way of working the same sum. About 300 calls a day at four minutes each, a fifth of them in the busy hour, comes to four erlangs. That is ten trunks at about one per cent blocking.
A channel count without a blocking assumption is half a number.
A small office with ordinary phone use lands at roughly one simultaneous call for every three to five people. Treat it as a sanity check rather than a measurement, because two shapes break it.
A trades or service business breaks it downwards. The office may sit in Melbourne’s south-east while the crews work out of Traralgon. Headcount says one thing and the busy hour says another, because most of that team is never on the office phones.
A reception-heavy business breaks it upwards. Calls arrive in bursts at a clinic at nine in the morning. The load is not spread evenly, and the busy hour is what you size for.
When every channel is in use, the next caller gets no explanation. They meet a busy tone, a queue or a diversion, and where your calls go after hours covers those settings in full.
Over-buying is the more common mistake, and nobody catches it. A quote written without a busy-hour figure is a guess, and the guess runs high.
Under-buying announces itself in a week: callers complain and somebody rings the provider. Over-buying makes no noise, and nobody is blamed for it. It stays on the bill for the life of the contract.
What a system costs is a separate question. The guide to what a business phone system costs holds the figures.
No, and it is a different question from how many channels you need. Handsets are about where people work; channels are about how many calls run at once.
A softphone is an app that turns a laptop or a mobile into an extension. It reaches a person on their own number without a handset on the desk.
Some desks still earn a physical phone. A reception desk that transfers calls all day is easier on real buttons. A shared bench where nobody has a computer needs one. So does a site where staff do not carry a company device.
Work through the desks one at a time rather than multiplying by the headcount.
Sizing a small business phone system this way takes about five minutes.
If the quote sits well above your figure, ask what the extra channels are for. If it sits below, ask what happens to the calls that do not fit.
The number only moves when the way the business works moves. Two more people on the phones, or a new contract that changes the morning rush, will shift it. Otherwise this week’s figure is good for the life of the contract. Work it out before you sign, not after the first month of busy signals.
Get the busy-hour figure off your current system before you take a quote. Then call Sixfam on 03 9200 2800 and start from that number. Buying business phone systems and networks to a measurement beats buying them to a guess.
Three or four simultaneous calls is an honest starting point for ten people with ordinary phone use. The published ranges allow a system of up to fifteen extensions on four calls at once. Measure your busiest hour before you commit to a number.
No. A number is an address that people dial, and a line is the path the call travels on. A business can hand a direct number to everybody and still run a small pool of channels.
It is one conversation in progress at a given moment. Five staff on five separate calls at once is five simultaneous calls, whoever they happen to be speaking to.
No. Extensions are internal numbers for people and handsets, and they share the channels rather than owning one each. Published vendor ranges put dozens of extensions on single-figure call counts.
Their call has nowhere to land until one of the channels frees up. Where it goes in the meantime is a setting, not a fixed behaviour. Getting the margin right above your busy hour is how you keep that from happening often.
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