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SIP trunk |
|---|
| Connects your phone system to the telephone network |
| Can contain multiple channels |
| Replaces the role of traditional physical phone lines |
| Usually configured around your wider communications requirements |
By Scott Aldridge, Director, PrimeTel · Updated September 2026
Key points
- A SIP trunk connects your existing phone system to the public telephone network over an internet connection.
- A SIP channel represents the capacity for one concurrent incoming or outgoing call.
- The number of SIP trunk channels you need is usually determined by peak simultaneous call demand, not simply your number of employees.
- One SIP trunk can support multiple channels, so businesses don’t generally need a separate trunk for every user or telephone number.
- Allowing some additional capacity for busy periods and future growth can prevent avoidable call congestion.
- The right setup should consider resilience, connectivity and future communications requirements as well as today’s call volumes.
If your business is considering SIP trunking, one of the first questions you’re likely to encounter is: how many SIP trunk channels do we actually need?
The answer isn’t necessarily tied to the number of employees. A 200-person business could need fewer channels than a 50-person business if its employees rarely make and receive calls at the same time.
What matters is concurrency: the number of calls your organisation needs to handle simultaneously during its busiest periods.
Getting this right helps you avoid two common problems. Too few channels can prevent callers from getting through or stop employees from making outbound calls. Too many, meanwhile, can leave you paying for capacity that rarely gets used.
So, before deciding how many channels per SIP trunk your organisation needs, it helps to understand how SIP trunks and channels work and how to assess the capacity your business needs.
What is a SIP trunk channel?
To understand what a SIP trunk channel is, it helps to separate the two parts.
A SIP trunk is the connection between your business phone system and the wider telephone network. It carries voice traffic using an internet connection and is commonly used by organisations that want to keep an existing compatible phone system while moving away from traditional telephone lines.
A SIP channel is the capacity within that trunk to handle one concurrent call.
Think of the SIP trunk as a motorway and the SIP trunk channels as its lanes. The motorway provides the connection, while the number of lanes determines how much traffic can travel along it at once.
For example, if you have 20 SIP trunk channels, your business can generally support up to 20 simultaneous calls through that capacity.
Those could be:
- 15 inbound and 5 outbound calls
- 10 inbound and 10 outbound calls
- 5 inbound and 15 outbound calls
- Any other combination up to the available concurrent call capacity
This is why the number of channels you need won’t necessarily match the number of employees or telephone numbers in your organisation.
SIP trunk vs SIP channel: what’s the difference?
The two terms are closely connected, which is why they can be confusing.
Put simply, the SIP trunk provides the connection to the telephone network, while the channels provide the capacity for calls within that connection.
When planning your SIP requirements, you need to look at how many concurrent calls your business is likely to handle rather than simply counting employees.
How many channels per SIP trunk can you have?
There isn’t one universal number of channels per SIP trunk.
Unlike older physical telephone services, where capacity was more closely tied to physical infrastructure, SIP trunking is much more flexible. A trunk can be configured with multiple channels according to the requirements of your organisation and the options available from your provider.
For example, one organisation might require 10 SIP trunk channels, while another might require 100 or more.
That flexibility allows channel capacity to be based on actual call usage rather than a fixed physical-line structure.
However, your internet connection also needs to be able to support the traffic generated by those calls. Channel capacity is only part of the overall picture, so it’s worth considering your business connectivity at the same time.
How many SIP trunk channels does my business need?
The starting point is your maximum number of simultaneous calls.
Imagine a professional services business with 100 employees. It would be easy to assume that it needs 100 SIP trunk channels. However, if call data shows that no more than 20 people are typically on external calls at the same time, 100 channels would be excessive.
On the other hand, a 100-person contact-heavy organisation where employees regularly make and receive external calls may need significantly more capacity.
A useful starting calculation is:
Required SIP channels = expected peak concurrent calls + appropriate spare capacity
For example:
These figures are illustrative rather than recommendations. Actual requirements depend on your call patterns, operating model, resilience requirements and how your communications environment is configured.
The important point is that channel requirements should be based on evidence rather than an arbitrary user-to-channel ratio.
What should you look at when calculating SIP channel requirements?
Historical call data is usually the best place to start.
Look at the periods when your telephone system is under the most pressure rather than your average day. Your organisation might average only ten simultaneous calls but regularly reach 25 at 9am on a Monday morning.
That peak is much more relevant when you’re deciding how many channels to purchase.
Other useful details to review include:
- Maximum concurrent inbound calls
- Maximum concurrent outbound calls
- Peak calling times
- Seasonal fluctuations
- Call queues
- Failed or blocked calls
- Changes in employee numbers
- Planned office expansion
- Future sales or customer service requirements
If your business operates across several locations, you should also establish whether channel capacity is being managed centrally or separately at each site.
This is where reviewing SIP trunks and channels alongside the rest of your communications setup becomes important. The capacity you choose needs to meet current demand while leaving enough room for the business to operate reliably as circumstances change.
Should I have spare SIP trunk channels?
Usually, yes.
Sizing your SIP channels exactly to your current maximum call volume leaves little room for unexpected demand.
If your highest recorded usage is 20 concurrent calls and you purchase exactly 20 channels, another call during a particularly busy period could exceed the available capacity.
The amount of headroom you need will depend on how predictable your call volumes are. A business with consistent demand may need relatively little additional capacity, while a customer-facing organisation with seasonal peaks may want more.
Think about events that could suddenly increase call volumes, including:
- Marketing campaigns
- Seasonal trading periods
- Service outages
- Product launches
- Customer service incidents
- Recruitment and expansion
- New offices or locations
Ultimately, the aim is to have enough spare capacity to deal with increased demand without paying for a large amount of capacity that will rarely be used.
Does internet bandwidth affect how many SIP channels I can use?
Yes. SIP calls travel across an IP connection, so your underlying connectivity needs to support the voice traffic being carried.
The exact bandwidth required for each call depends on factors such as the codec being used and your network configuration. This means channel numbers alone don’t give you a complete picture of your connectivity requirements.
Quality of Service (QoS), latency, jitter, packet loss and network resilience can all affect call performance.
For an organisation that relies heavily on its telephone system to handle business-critical calls, these are the questions you should also be asking:
- Can our current connection support the required voice traffic?
- How is voice traffic prioritised?
- What happens if our primary connection fails?
- Do we have appropriate network resilience?
- How will future growth affect bandwidth requirements?
A well-sized SIP trunk still depends on the network underneath it. If the connection can’t reliably handle the traffic, call quality and availability can suffer.
PrimeTel’s connectivity solutions can help businesses assess their voice and network requirements together rather than treating them as completely separate decisions.
What happens if I don’t have enough SIP trunk channels?
When all available SIP channels are being used, additional calls may be unable to connect until capacity becomes available.
Exactly what the caller or employee experiences will depend on your configuration and provider, but the impact can be significant.
A failed inbound call could mean a missed sales opportunity, while a potential customer unable to reach your team may get frustrated and find an alternative route to get help.
The issue can also be easy to miss if it only happens during short periods of peak demand. Your telephone system may appear to work perfectly for most of the day but struggle during the busiest 30 minutes.
That’s why looking at peak concurrency and historical call data is much more useful than relying solely on average usage.
Can I add more SIP channels later?
One of the advantages of SIP trunking is that it can scale with your business.
Subject to your provider, contract and underlying infrastructure, additional SIP trunk channels can generally be added without installing new physical telephone lines.
That can be useful for growing businesses or organisations where call requirements fluctuate throughout the year.
However, don’t assume that scaling will always be completely frictionless. Before choosing a provider, find out how the process works and check:
- How quickly channels can be added
- Whether additional capacity changes your contract
- How additional channels are priced
- Whether there are minimum commitments
- Whether your existing connectivity can support the extra traffic
- What happens if you need to reduce capacity later
The cheapest initial proposal isn’t necessarily the most commercially attractive once your organisation’s requirements change.
SIP trunking or hosted telephony?
SIP trunking isn’t automatically the right communications strategy for every organisation.
If you already have an on-premise or compatible PBX that still meets your needs, SIP trunking can provide a way to modernise external voice connectivity without replacing the entire telephone system.
Other businesses may benefit from moving towards a cloud-based hosted telephony solution, particularly if they want to reduce their reliance on on-premise equipment or support hybrid working.
Larger customer-facing operations may also need to consider whether their requirements extend beyond business telephony into Contact Centre as a Service (CCaaS).
The most suitable option will depend on what you’re trying to achieve rather than which technology happens to be newest.
How PrimeTel helps businesses review SIP trunking
Calculating your required SIP trunk capacity is relatively straightforward once you have reliable call data. Choosing the right provider and assessing the wider commercial and technical requirements can take more work.
At PrimeTel, we help organisations buy business-critical technology through a structured, competitive sourcing process.
We’re not tied to one SIP trunking provider, and we’re not here to push a particular platform. Instead, we help define what the business actually needs, engage suitable suppliers and compare the available options on a level playing field.
That can include reviewing:
- Existing call volumes and capacity
- SIP trunk channels
- Connectivity and resilience
- Supplier service levels
- Pricing and commercial structures
- Contract terms
- Future scalability
- Migration requirements
- Wider telephony requirements
We also remain involved through evaluation, negotiation and implementation, helping make sure the solution selected during the buying process works once the technology goes live.