Quick answer
A line or channel is capacity for one call at a time. An extension is an internal address. A DDI is a public number pointing at one of them. Only the first costs you money per unit, and you should size it on how many calls happen simultaneously at your busiest hour - which in most offices is a fraction of the headcount. A ten-person office often runs comfortably on three or four channels; a four-person sales team can need more than four.
Getting this wrong in either direction is expensive. Too few and callers hear an engaged tone, which on a copper line took weeks to fix. Too many and you pay for silence. This guide gives you the vocabulary and, more usefully, a method for working out the number rather than guessing at it. If you would rather we measured it from your actual call records, get a no-obligation quote.
The four terms, pinned down
| Term | What it is | How many you need | What it costs |
|---|---|---|---|
| Line or channel | One simultaneous call path | Based on peak concurrency | The variable cost - this is the meter |
| Extension | An internal short number for a phone or user | One per phone or person | Nothing directly |
| DDI | A public number reaching a specific destination | As many as you want to publish | Cheap, usually per block |
| Seat or user | A named person on a hosted system | One per person who answers calls | The main cost on hosted platforms |
The distinction that matters: channels limit how many calls can happen at once; extensions and DDIs decide where those calls go. You can have fifty DDIs and four channels. You can have twenty extensions and three channels. Nothing about handing out direct numbers consumes capacity - a fact worth knowing before somebody quotes you a channel per employee. The numbering side is covered in what is DDI.
How to work out how many channels you need
You do not have to guess. Two numbers give you a defensible starting point, and you can get both from your phone bill or your provider.
Step 1 - find your busiest hour. Count the calls handled in that hour, inbound and outbound together.
Step 2 - find your average call length, in minutes.
Step 3 - multiply and divide by 60. That gives you your average number of simultaneous calls during the peak hour.
An illustrative worked example: 45 calls in the busiest hour, averaging 4 minutes each. That is 180 call-minutes in a 60-minute hour, so 180 รท 60 = 3 simultaneous calls on average.
Step 4 - add headroom, because calls do not arrive evenly. An average of three means there will be moments at five. As a practical rule, take the average figure and roughly double it for a small business with bursty demand, or add about half again where call arrival is steady. In the example above that lands you at five or six channels, not three.
If blocked calls genuinely cost you sales, ask your provider to run a proper Erlang calculation against a target blocking rate - it is a standard piece of telephony maths and any decent provider can do it in minutes. But the four steps above will get you within one channel of the right answer, which is enough to stop you buying twelve.
Three business shapes, sized
| Business | Headcount | What is actually happening | Channels, as a guide |
|---|---|---|---|
| Ten-person professional services office | 10 | Scheduled calls, low volume, most staff on email | 3 to 4 |
| Four-person sales team, outbound | 4 | Everybody on the phone most of the day, plus inbound returns | 5 to 6 |
| Twenty-person warehouse and trade counter | 20 | Two people answer the phone; the rest never do | 3 to 4 |
| Six-person bookings line with a queue | 6 | Callers held in a queue occupy capacity while waiting | 8 or more |
The bookings row is the one that catches people out, and it is the exception to every rule of thumb. A caller sitting in a queue is using a channel the entire time they wait. If your business queues callers, your channel count has to cover the people waiting as well as the people talking, and that pushes the number above headcount rather than below it.
What consumes a channel, and what does not
| Consumes capacity | Does not consume capacity |
|---|---|
| An active call, inbound or outbound | An extension sitting idle |
| A caller waiting in a queue | A DDI number nobody is dialling |
| Each leg of a conference call | A hunt group ringing five phones for one call |
| A call on hold at your end | Voicemail already recorded |
| A call diverted out to an external mobile - often two legs | An internal call between two extensions on the same system |
Two rows deserve a note. A hunt group ringing five phones is still one call, so hunting does not multiply capacity - a genuinely common misunderstanding. And diverting a call out to an external mobile can occupy two channels at once, because the call comes in on one and goes back out on another. Businesses that divert heavily out of hours should size for that.
Where the model changes: hosted systems
Everything above assumes you are buying channels, which is how SIP trunks into an on-site phone system work. On a hosted platform the commercial model is usually different: you buy a seat per person, and concurrency is handled by the platform rather than metered to you.
That is a genuine simplification, and it is one of the underrated reasons hosted suits businesses that cannot predict their peak. It also changes what you should scrutinise on a quote - the question stops being "how many channels?" and becomes "how many people genuinely need a seat, and what does each tier include?" Not everyone needs a full seat; a warehouse phone that only takes internal calls may not need one at all. The two models are compared in SIP trunking vs hosted VoIP, and the wider choice of replacement in business landline alternatives.
Sizing the connection underneath
Channels are only useful if the internet connection can carry them. As a planning figure, allow roughly 100 kbps in each direction per simultaneous call on an uncompressed codec - so eight concurrent calls is under a megabit each way, which is trivial for any modern connection.
The number is small enough that raw speed is almost never the problem. What matters instead is consistency and contention: whether voice traffic is prioritised over a large file upload, and whether the connection's upload is generous enough that a busy afternoon does not squeeze it. Our guide to internet speed for VoIP covers what to check, and the SoGEA vs FTTP comparison covers the upload difference between the two connection types.
What the switch-off changes about all this
On copper, every channel was a physical thing: rented monthly, ordered weeks in advance, and expensive enough that businesses systematically under-bought and lived with engaged tones. ISDN30 sold capacity in blocks, so sites ended up renting round numbers rather than needed ones.
Three things change once capacity is software:
You can be wrong cheaply. Adding a channel takes minutes. That removes the reason to over-order defensively.
You can flex seasonally. A business with a Christmas peak can add capacity in November and remove it in January.
Idle capacity becomes visible. Nobody will tell you that you are paying for channels you never use, so put a note in the diary to check your peak concurrency once a year against what you are buying.
The bottom line
The jargon is worse than the concept. Channels are simultaneous calls and the only thing you meter; extensions and DDIs are routing and effectively free. Work out your peak-hour concurrency from call volume and duration, add headroom for burstiness, add more if you queue callers, and check the connection can carry it. Then buy that, not a channel per desk - and review it once a year, because the whole point of leaving copper behind is that being wrong no longer costs you six weeks.
Want the number calculated from your actual call records rather than a rule of thumb? Get a no-obligation quote or call 0333 006 9399, and we will size it before we price it.
Getting unknown calls on a business line? Our free UK phone number checker shows which network or provider Ofcom allocated any number to, and whether the range is in service — no sign-up.
Frequently asked questions
What is the difference between a phone line and a channel?
Practically nothing - both mean one path for one call at a time. "Line" is the analogue term, "channel" is used on ISDN and internet-based services. Two channels means two calls can happen simultaneously, regardless of how many phones or numbers you have.
How many phone lines does my business need?
Size on peak concurrency, not headcount. Take the number of calls in your busiest hour, multiply by average call length in minutes, divide by 60 for your average simultaneous calls, then add headroom for burstiness. Most small offices need noticeably fewer channels than they have people.
What is the difference between an extension and a DDI?
An extension is an internal short number your staff dial to reach each other. A DDI is a public number that the outside world can dial to reach a specific person, team or function directly. A DDI usually points at an extension, but they are different things and only one is visible to customers.
Does giving everyone a direct number mean I need more lines?
No. DDIs share your underlying call capacity, which is the entire reason they exist. You can publish fifty direct numbers and still only need enough channels for the handful of calls happening at once. Numbers are cheap; capacity is what you meter.
Do calls waiting in a queue use up channels?
Yes, and this is the biggest exception to the usual sizing rules. A caller on hold in a queue occupies capacity for the whole time they wait, so businesses that queue callers need to size for the people waiting as well as the people talking - often more channels than staff.
How much bandwidth does each call need?
As a planning figure, roughly 100 kbps in each direction per simultaneous call on an uncompressed codec. That makes raw speed a non-issue on any modern connection. What matters is consistency, upload headroom and whether voice traffic is prioritised over large transfers.
Is it easier to add lines on a modern system?
Far easier. Channels are a software setting that can be added or removed in minutes without an engineer, so you can size to your real peak instead of over-ordering defensively. It also means you should review your capacity annually, because nobody will point out that you are paying for channels you never use.