A SIP channel is one simultaneous phone call. If you have ten channels, ten calls can be live at once, and the eleventh caller hears a busy tone. Getting that number right is one of the most useful decisions you’ll make when buying a SIP trunk for business.

It’s also becoming urgent. The UK’s legacy phone network is due to be retired on 31 January 2027, and Openreach has said it will not be delayed again. Firms moving off ISDN are being asked how many channels they want, often before they’ve worked it out.

This guide shows how to size a trunk sensibly, without guesswork.

What a Channel Actually Is

Think of channels as lanes on a motorway. Each live call, inbound or outbound, takes one lane for as long as it lasts. It doesn’t matter how many staff you have or how many numbers you own. Only the number of calls happening at the same moment counts.

Two points trip people up:

• Inbound and outbound share the same pool. Ten channels means ten calls in total, in any mix.

• Numbers aren’t channels. You can have fifty direct dial numbers on a ten-channel trunk. Numbers decide where calls go, channels decide how many can happen at once.

If you’re replacing ISDN, your current lines give you a ceiling, not a target. An ISDN2 line carries two simultaneous calls. An ISDN30 line carries up to thirty. Many businesses pay for far more capacity than they ever use, because ISDN was sold in fixed blocks.

Before you copy those numbers across, find out what you actually use.

Step One: Find Your Peak

Ask your current provider or PBX for call logs covering at least a month, preferably a busy one. You’re looking for the peak number of simultaneous calls and when it happens.

Many PBX systems report this directly as “maximum concurrent calls”. If yours doesn’t, a provider or IT supplier can usually work it out from call records.

Look at patterns, not just the single highest spike. A one-off rush on a bad day matters less than a regular Monday morning peak.

Step Two: Add the Hidden Calls

Raw logs often miss situations where one caller uses more than one channel. Count these before you size.

Forwarded and transferred calls. If an incoming call is forwarded to a mobile, it uses two channels: one for the caller, one for the forward.

Ring groups that dial external numbers. If a call rings several outside mobiles at once, each leg can use a channel while it rings.

Outbound campaigns. Sales teams dialling in parallel can lift your peak well beyond headcount.

Auto-dialled or alarm calls. Alarms, fax lines and card machines may use channels too.

Remote and hybrid working. Staff calling from mobile apps still use trunk channels when calling external numbers.

Step Three: Add a Sensible Buffer

Once you know your busiest realistic peak, add headroom for growth and surprises. A common approach is to add a modest margin above peak, then review after a few months.

If you want to be more precise, telecoms engineers use a formula called Erlang B to estimate how many channels you need for a given chance of callers hearing a busy tone.

A Simple Worked Example

Say a 30-person accountancy firm sees 120 calls in its busiest hour, averaging four minutes each.

• 120 calls × 4 minutes = 480 call-minutes in an hour

• 480 ÷ 60 = 8 hours of traffic, which engineers call 8 Erlangs

For a 1% chance of a busy tone at that peak, Erlang B tables suggest about 15 channels. For a looser target, you might manage with slightly fewer. That’s well below the 30 channels an ISDN30 line would have given them.

Treat this as an estimate. Your provider should be able to run the numbers with you, and many online Erlang calculators are free.

What Happens If You Get It Wrong

Too few channels. Callers hear a busy tone or fail to connect during peaks. Customers rarely try twice. It’s one of the most costly mistakes, because you don’t see the calls you’ve lost.

Too many channels. You pay for capacity you don’t use. It’s usually cheaper to start sensibly and add channels than to overbuy, provided your provider lets you scale quickly.

Ask each SIP provider in the UK you speak to how fast channels can be added, whether there’s a charge and whether short-term burst capacity is offered.

Channels and Bandwidth

Every active call needs bandwidth on your internet connection, in both directions. As a rough planning figure, uncompressed voice (G.711) needs somewhere between 80 and 100 kbps each way per call once overhead is included, and compressed codecs use less. Ten simultaneous calls would therefore need around one megabit each way, before anything else on the network.

That’s a small slice of a fibre connection, but it matters on busy, shared lines. Two things help:

• Voice prioritisation on your router, so downloads and backups don’t crowd out calls

• A connection suited to the load, such as business fibre or a dedicated line for larger or voice-heavy sites

An IP SIP trunk is only as good as the connection beneath it. A provider that asks about your network before quoting is showing the right instincts.

Multiple Sites and Shared Channels

If you have several offices, ask whether channels can be pooled across sites. Branches often peak at different times, so one shared pool can be cheaper than separate allocations. Not every provider offers this, so check before you plan.

Comparing Quotes Fairly

Once you have your number, give every provider the same brief. Quotes become comparable only when they’re based on the same channel count, numbers, call patterns and resilience needs.

Look beyond the channel rental. A realistic quote also covers number fees, call charges for mobiles, international and 03/08 numbers, setup costs and contract terms.

You’ll see lists of the best SIP providers in the UK and the best UK VoIP providers online. Use them as a starting point at most, because they can’t know your call volumes or sites. Equally, searching for the cheapest UK VoIP providers can lead to a low channel price that hides expensive calls or weak support. The right comparison is total cost against your real usage, plus resilience and service.

Resilience Matters More With Fewer Channels

Rightsizing doesn’t mean cutting corners on reliability. If your connection drops, every channel stops. Ask about automatic call diversion to mobile numbers, a second connection for important sites and battery backup for your router. Also check what else depends on your phone lines, including alarms, lift emergency lines, CCTV links and card payment terminals.

A Realistic Example

A 45-person engineering firm in Sheffield has an ISDN30 line and assumes it needs thirty channels. Their PBX logs show a peak of eleven simultaneous calls, with a few Monday-morning spikes to thirteen when several calls are forwarded to engineers’ mobiles.

They add forwarded legs to their count and choose sixteen channels, with a provider who can add more within a day. After three months, usage review shows a maximum of fourteen. They stay at sixteen, and pay for roughly half the capacity of their old setup.

A Simple Checklist

1. Pull at least a month of call logs and find peak concurrent calls

2. Add forwarded, transferred and ring-group legs

3. Add a modest buffer for growth

4. Check bandwidth and router prioritisation

5. Ask providers how quickly channels can be changed

6. Review usage after the first few months

Wavetel Business is one example of a UK provider you might talk to about sizing, and any SIP trunk service should be willing to review your call data and put their recommendation in writing.

Frequently Asked QuestionsWhat is a SIP channel?

It’s one simultaneous call on a SIP trunk, inbound or outbound. Ten channels allow ten calls at the same time.

How many channels do I need per employee?

There’s no fixed ratio. It depends on how heavily your team uses the phone. A call-heavy sales team needs far more per head than a small design studio.

Is a SIP trunk the same as a phone number?

No. Numbers decide where calls go, while channels decide how many calls can happen at once.

Can I add channels later?

Usually, yes. Ask your provider how quickly it can be done, and whether there’s a cost.

Is SIP trunking for business cheaper than ISDN?

Often it is, partly because you can size channels more precisely. Compare total cost, including calls and numbers, rather than channel price alone.

Conclusion

Choosing the right number of SIP channels comes down to knowing your real peak, counting the hidden calls and adding a sensible buffer. Headcount and ISDN line sizes are poor guides.

The practical takeaway is to pull your call logs this week, work out peak concurrent calls and give every provider the same number. Do it well before January 2027 so there’s time to compare quotes calmly.